Serve Prometheus metrics at /metrics behind basic auth (closes #94)

With METRICS_USERNAME and METRICS_PASSWORD both set, the backend records request metrics through go-http-metrics in a registry of its own, with Go's runtime and process metrics, and serves them at GET /metrics behind basic auth; nginx passes /metrics to it. With neither set there is no such route; one alone, or a METRICS_USERNAME containing ":", stops the start with an error naming the setting. Only requests that reach the health check or POST /api/v1/reports are recorded, as the labels are path and method, which clients could otherwise make up without end; POST /api/v1/reports is registered by its full path for that. Deviation: go get and go mod tidy ran directly; no entrypoint added a Go dependency yet (issue #45). Model: opus-5-5
2026-10-04 04:58:47 +02:00
parent d412815953
commit 81d4153e78
12 changed files with 306 additions and 33 deletions
@@ -88,6 +88,8 @@ project layout:
 | `TRUSTED_PROXIES`      | loopback + RFC1918   | Comma-separated CIDRs whose `X-Forwarded-For` / `X-Real-IP` headers are trusted for client IP resolution |
 | `REPORTS_PER_MINUTE`   | `60`                 | Reports each client address may send a minute; see [Report limits](#report-limits)                       |
 | `CORS_ALLOWED_ORIGINS` | empty                | Comma-separated origins whose pages may call the API; see [CORS](#cors)                                  |
+| `METRICS_USERNAME`     | empty                | Basic auth user name for `/metrics`; see [Metrics](#metrics)                                             |
+| `METRICS_PASSWORD`     | empty                | Basic auth password for `/metrics`; see [Metrics](#metrics)                                              |

 `TRUSTED_PROXIES` defaults to
 `127.0.0.1/32,::1/128,10.0.0.0/8,172.16.0.0/12,192.168.0.0/16`. The loopback
@@ -103,16 +105,16 @@ starting, with an error naming the variable. An empty variable counts as unset.
 ### Container image

 The root `Dockerfile` builds one image in which nginx listens on the public port
-8080, serves the frontend, and proxies `/api/` and `/.well-known/healthcheck` to
-this server. The image's entrypoint, `bin/entrypoint.sh`, starts the server as
-user `netwatch` (uid 1000) with `BIND_ADDRESS=127.0.0.1` and `PORT=8081`, so
-only nginx reaches it, and with `TRUSTED_PROXIES=127.0.0.1/32`, so it takes the
-client address nginx passes on and no other. `DATA_DIR` is `/data/reports`, on
-the `/data` volume; before starting the server, the entrypoint creates it and
-gives it and `/data` to `netwatch` with `netwatch-server prepare-data-dir`,
-which acts on nothing outside `/data`. nginx replaces the security headers this
-server sets with those in the root `security-headers.conf`, so those are what
-clients of the image see.
+8080, serves the frontend, and proxies `/api/`, `/.well-known/healthcheck` and
+`/metrics` to this server. The image's entrypoint, `bin/entrypoint.sh`, starts
+the server as user `netwatch` (uid 1000) with `BIND_ADDRESS=127.0.0.1` and
+`PORT=8081`, so only nginx reaches it, and with `TRUSTED_PROXIES=127.0.0.1/32`,
+so it takes the client address nginx passes on and no other. `DATA_DIR` is
+`/data/reports`, on the `/data` volume; before starting the server, the
+entrypoint creates it and gives it and `/data` to `netwatch` with
+`netwatch-server prepare-data-dir`, which acts on nothing outside `/data`. nginx
+replaces the security headers this server sets with those in the root
+`security-headers.conf`, so those are what clients of the image see.

 The container's own `TRUSTED_PROXIES` goes to nginx instead: IP addresses or
 CIDRs, separated by commas, of the reverse proxies in front of the container.
@@ -176,6 +178,26 @@ origin, `scheme://host` with an optional `:port`, as browsers send it: no path,
 not even a trailing `/`, and no `*`. Any other entry stops the server from
 starting, with an error naming `CORS_ALLOWED_ORIGINS`.

+### Metrics
+
+With both `METRICS_USERNAME` and `METRICS_PASSWORD` set, the server serves
+Prometheus metrics at `GET /metrics` to requests with those as their basic auth
+credentials, and answers any other with 401. For each request that reaches the
+health check or `POST /api/v1/reports`, those the rate limit refuses included,
+the metrics record its duration and response size, labelled with its path,
+method and status; they also count those requests in progress, and include Go's
+runtime and process metrics. No other request is recorded: not those to
+`/metrics` itself, and not those answered before they reach either route, such
+as a CORS preflight, or a request refused with 404 for a path no route has, 405
+for a method its route does not take, or 413 for declaring a body length over
+the 1 MiB limit. A report whose body goes over the limit without declaring its
+length reaches the route, is answered 413 there, and is recorded with that
+status. Clients can make up any number of paths and methods, and each would add
+labels to the metrics for as long as the server runs. With neither set, nothing
+is recorded and `/metrics` answers 404. One without the other stops the server
+from starting, with an error naming both; so does a `METRICS_USERNAME`
+containing `:`, which basic auth cannot carry, with an error naming it.
+
 ## TODO

 - Add integration test that POSTs a report and verifies the compressed output
@@ -3,20 +3,29 @@ module sneak.berlin/go/netwatch
 go 1.25.5

 require (
+	github.com/99designs/basicauth-go v0.0.0-20230316000542-bf6f9cbbf0f8
 	github.com/go-chi/chi/v5 v5.2.5
 	github.com/go-chi/cors v1.2.2
 	github.com/go-chi/httprate v0.16.0
 	github.com/joho/godotenv v1.5.1
-	github.com/klauspost/compress v1.18.4
+	github.com/klauspost/compress v1.19.1
+	github.com/prometheus/client_golang v1.24.1
+	github.com/slok/go-http-metrics v0.13.0
 	github.com/spf13/viper v1.21.0
 	go.uber.org/fx v1.24.0
 )

 require (
+	github.com/beorn7/perks v1.0.1 // indirect
+	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/fsnotify/fsnotify v1.9.0 // indirect
 	github.com/go-viper/mapstructure/v2 v2.4.0 // indirect
 	github.com/klauspost/cpuid/v2 v2.2.10 // indirect
+	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
 	github.com/pelletier/go-toml/v2 v2.2.4 // indirect
+	github.com/prometheus/client_model v0.6.2 // indirect
+	github.com/prometheus/common v0.70.1 // indirect
+	github.com/prometheus/procfs v0.21.1 // indirect
 	github.com/sagikazarmark/locafero v0.11.0 // indirect
 	github.com/sourcegraph/conc v0.3.1-0.20240121214520-5f936abd7ae8 // indirect
 	github.com/spf13/afero v1.15.0 // indirect
@@ -28,6 +37,7 @@ require (
 	go.uber.org/multierr v1.10.0 // indirect
 	go.uber.org/zap v1.26.0 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
-	golang.org/x/sys v0.30.0 // indirect
-	golang.org/x/text v0.28.0 // indirect
+	golang.org/x/sys v0.47.0 // indirect
+	golang.org/x/text v0.40.0 // indirect
+	google.golang.org/protobuf v1.36.11 // indirect
 )
@@ -1,3 +1,9 @@
+github.com/99designs/basicauth-go v0.0.0-20230316000542-bf6f9cbbf0f8 h1:nMpu1t4amK3vJWBibQ5X/Nv0aXL+b69TQf2uK5PH7Go=
+github.com/99designs/basicauth-go v0.0.0-20230316000542-bf6f9cbbf0f8/go.mod h1:3cARGAK9CfW3HoxCy1a0G4TKrdiKke8ftOMEOHyySYs=
+github.com/beorn7/perks v1.0.1 h1:VlbKKnNfV8bJzeqoa4cOKqO6bYr3WgKZxO8Z16+hsOM=
+github.com/beorn7/perks v1.0.1/go.mod h1:G2ZrVWU2WbWT9wwq4/hrbKbnv/1ERSJQ0ibhJ6rlkpw=
+github.com/cespare/xxhash/v2 v2.3.0 h1:UL815xU9SqsFlibzuggzjXhog7bL6oX9BbNZnL2UFvs=
+github.com/cespare/xxhash/v2 v2.3.0/go.mod h1:VGX0DQ3Q6kWi7AoAeZDth3/j3BFtOZR5XLFGgcrjCOs=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/frankban/quicktest v1.14.6 h1:7Xjx+VpznH+oBnejlPUj8oUpdxnVs4f8XU8WnHkI4W8=
@@ -12,26 +18,40 @@ github.com/go-chi/httprate v0.16.0 h1:8V5DH9j6pSK6UQoBsTpvMyFxycqaKEIToyPKzHJjUa
 github.com/go-chi/httprate v0.16.0/go.mod h1:A8lo+qRhk+s9LiuP5saS7XCGDXRXMcrueq0NfIuCa/I=
 github.com/go-viper/mapstructure/v2 v2.4.0 h1:EBsztssimR/CONLSZZ04E8qAkxNYq4Qp9LvH92wZUgs=
 github.com/go-viper/mapstructure/v2 v2.4.0/go.mod h1:oJDH3BJKyqBA2TXFhDsKDGDTlndYOZ6rGS0BRZIxGhM=
-github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
-github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
+github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
+github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX3N/iU=
 github.com/joho/godotenv v1.5.1 h1:7eLL/+HRGLY0ldzfGMeQkb7vMd0as4CfYvUVzLqw0N0=
 github.com/joho/godotenv v1.5.1/go.mod h1:f4LDr5Voq0i2e/R5DDNOoa2zzDfwtkZa6DnEwAbqwq4=
-github.com/klauspost/compress v1.18.4 h1:RPhnKRAQ4Fh8zU2FY/6ZFDwTVTxgJ/EMydqSTzE9a2c=
-github.com/klauspost/compress v1.18.4/go.mod h1:R0h/fSBs8DE4ENlcrlib3PsXS61voFxhIs2DeRhCvJ4=
+github.com/klauspost/compress v1.19.1 h1:VsB4HPswih7mmZ8WleSFQ75c/Ui1M4trX5oAsJnhSlk=
+github.com/klauspost/compress v1.19.1/go.mod h1:cwPg85FWrGar70rWktvGQj8/hthj3wpl0PGDogxkrSQ=
 github.com/klauspost/cpuid/v2 v2.2.10 h1:tBs3QSyvjDyFTq3uoc/9xFpCuOsJQFNPiAhYdw2skhE=
 github.com/klauspost/cpuid/v2 v2.2.10/go.mod h1:hqwkgyIinND0mEev00jJYCxPNVRVXFQeu1XKlok6oO0=
 github.com/kr/pretty v0.3.1 h1:flRD4NNwYAUpkphVc1HcthR4KEIFJ65n8Mw5qdRn3LE=
 github.com/kr/pretty v0.3.1/go.mod h1:hoEshYVHaxMs3cyo3Yncou5ZscifuDolrwPKZanG3xk=
 github.com/kr/text v0.2.0 h1:5Nx0Ya0ZqY2ygV366QzturHI13Jq95ApcVaJBhpS+AY=
 github.com/kr/text v0.2.0/go.mod h1:eLer722TekiGuMkidMxC/pM04lWEeraHUUmBw8l2grE=
+github.com/kylelemons/godebug v1.1.0 h1:RPNrshWIDI6G2gRW9EHilWtl7Z6Sb1BR0xunSBf0SNc=
+github.com/kylelemons/godebug v1.1.0/go.mod h1:9/0rRGxNHcop5bhtWyNeEfOS8JIWk580+fNqagV/RAw=
+github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 h1:C3w9PqII01/Oq1c1nUAm88MOHcQC9l5mIlSMApZMrHA=
+github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822/go.mod h1:+n7T8mK8HuQTcFwEeznm/DIxMOiR9yIdICNftLE1DvQ=
 github.com/pelletier/go-toml/v2 v2.2.4 h1:mye9XuhQ6gvn5h28+VilKrrPoQVanw5PMw/TB0t5Ec4=
 github.com/pelletier/go-toml/v2 v2.2.4/go.mod h1:2gIqNv+qfxSVS7cM2xJQKtLSTLUE9V8t9Stt+h56mCY=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/prometheus/client_golang v1.24.1 h1:JnJkREXzWxUdCuPFpIWZiPispT9xVV59uiuyR2bPlnU=
+github.com/prometheus/client_golang v1.24.1/go.mod h1:F+oSRECHg4sse5ucfYpYDeIv/hu68Zo0uoHKetWnzcE=
+github.com/prometheus/client_model v0.6.2 h1:oBsgwpGs7iVziMvrGhE53c/GrLUsZdHnqNwqPLxwZyk=
+github.com/prometheus/client_model v0.6.2/go.mod h1:y3m2F6Gdpfy6Ut/GBsUqTWZqCUvMVzSfMLjcu6wAwpE=
+github.com/prometheus/common v0.70.1 h1:1HvjP4D5oL3t8RsPlwxA9onvvStjtIHYE5XuuwOi/PY=
+github.com/prometheus/common v0.70.1/go.mod h1:VdFUQDMZK3VLkurFUVhia6uys/0suUp86TJz5qbJRhc=
+github.com/prometheus/procfs v0.21.1 h1:GljZCt+zSTS+NZq88cyQ1LjZ+RCHp3uVuabBWA5+OJI=
+github.com/prometheus/procfs v0.21.1/go.mod h1:aB55Cww9pdSJVHk0hUf0inxWyyjPogFIjmHKYgMKmtY=
 github.com/rogpeppe/go-internal v1.9.0 h1:73kH8U+JUqXU8lRuOHeVHaa/SZPifC7BkcraZVejAe8=
 github.com/rogpeppe/go-internal v1.9.0/go.mod h1:WtVeX8xhTBvf0smdhujwtBcq4Qrzq/fJaraNFVN+nFs=
 github.com/sagikazarmark/locafero v0.11.0 h1:1iurJgmM9G3PA/I+wWYIOw/5SyBtxapeHDcg+AAIFXc=
 github.com/sagikazarmark/locafero v0.11.0/go.mod h1:nVIGvgyzw595SUSUE6tvCp3YYTeHs15MvlmU87WwIik=
+github.com/slok/go-http-metrics v0.13.0 h1:lQDyJJx9wKhmbliyUsZ2l6peGnXRHjsjoqPt5VYzcP8=
+github.com/slok/go-http-metrics v0.13.0/go.mod h1:HIr7t/HbN2sJaunvnt9wKP9xoBBVZFo1/KiHU3b0w+4=
 github.com/sourcegraph/conc v0.3.1-0.20240121214520-5f936abd7ae8 h1:+jumHNA0Wrelhe64i8F6HNlS8pkoyMv5sreGx2Ry5Rw=
 github.com/sourcegraph/conc v0.3.1-0.20240121214520-5f936abd7ae8/go.mod h1:3n1Cwaq1E1/1lhQhtRK2ts/ZwZEhjcQeJQ1RuC6Q/8U=
 github.com/spf13/afero v1.15.0 h1:b/YBCLWAJdFWJTN9cLhiXXcD7mzKn9Dm86dNnfyQw1I=
@@ -42,6 +62,8 @@ github.com/spf13/pflag v1.0.10 h1:4EBh2KAYBwaONj6b2Ye1GiHfwjqyROoF4RwYO+vPwFk=
 github.com/spf13/pflag v1.0.10/go.mod h1:McXfInJRrz4CZXVZOBLb0bTZqETkiAhM9Iw0y3An2Bg=
 github.com/spf13/viper v1.21.0 h1:x5S+0EU27Lbphp4UKm1C+1oQO+rKx36vfCoaVebLFSU=
 github.com/spf13/viper v1.21.0/go.mod h1:P0lhsswPGWD/1lZJ9ny3fYnVqxiegrlNrEmgLjbTCAY=
+github.com/stretchr/objx v0.5.2 h1:xuMeJ0Sdp5ZMRXx/aWO6RZxdr3beISkG5/G/aIRr3pY=
+github.com/stretchr/objx v0.5.2/go.mod h1:FRsXN1f5AsAjCGJKqEizvkpNtU+EGNCLh3NxZ/8L+MA=
 github.com/stretchr/testify v1.11.1 h1:7s2iGBzp5EwR7/aIZr8ao5+dra3wiQyKjjFuvgVKu7U=
 github.com/stretchr/testify v1.11.1/go.mod h1:wZwfW3scLgRK+23gO65QZefKpKQRnfz6sD981Nm4B6U=
 github.com/subosito/gotenv v1.6.0 h1:9NlTDc1FTs4qu0DDq7AEtTPNw6SVm7uBMsUCUjABIf8=
@@ -54,18 +76,22 @@ go.uber.org/dig v1.19.0 h1:BACLhebsYdpQ7IROQ1AGPjrXcP5dF80U3gKoFzbaq/4=
 go.uber.org/dig v1.19.0/go.mod h1:Us0rSJiThwCv2GteUN0Q7OKvU7n5J4dxZ9JKUXozFdE=
 go.uber.org/fx v1.24.0 h1:wE8mruvpg2kiiL1Vqd0CC+tr0/24XIB10Iwp2lLWzkg=
 go.uber.org/fx v1.24.0/go.mod h1:AmDeGyS+ZARGKM4tlH4FY2Jr63VjbEDJHtqXTGP5hbo=
-go.uber.org/goleak v1.2.0 h1:xqgm/S+aQvhWFTtR0XK3Jvg7z8kGV8P4X14IzwN3Eqk=
-go.uber.org/goleak v1.2.0/go.mod h1:XJYK+MuIchqpmGmUSAzotztawfKvYLUIgg7guXrwVUo=
+go.uber.org/goleak v1.3.0 h1:2K3zAYmnTNqV73imy9J1T3WC+gmCePx2hEGkimedGto=
+go.uber.org/goleak v1.3.0/go.mod h1:CoHD4mav9JJNrW/WLlf7HGZPjdw8EucARQHekz1X6bE=
 go.uber.org/multierr v1.10.0 h1:S0h4aNzvfcFsC3dRF1jLoaov7oRaKqRGC/pUEJ2yvPQ=
 go.uber.org/multierr v1.10.0/go.mod h1:20+QtiLqy0Nd6FdQB9TLXag12DsQkrbs3htMFfDN80Y=
 go.uber.org/zap v1.26.0 h1:sI7k6L95XOKS281NhVKOFCUNIvv9e0w4BF8N3u+tCRo=
 go.uber.org/zap v1.26.0/go.mod h1:dtElttAiwGvoJ/vj4IwHBS/gXsEu/pZ50mUIRWuG0so=
+go.yaml.in/yaml/v2 v2.4.4 h1:tuyd0P+2Ont/d6e2rl3be67goVK4R6deVxCUX5vyPaQ=
+go.yaml.in/yaml/v2 v2.4.4/go.mod h1:gMZqIpDtDqOfM0uNfy0SkpRhvUryYH0Z6wdMYcacYXQ=
 go.yaml.in/yaml/v3 v3.0.4 h1:tfq32ie2Jv2UxXFdLJdh3jXuOzWiL1fo0bu/FbuKpbc=
 go.yaml.in/yaml/v3 v3.0.4/go.mod h1:DhzuOOF2ATzADvBadXxruRBLzYTpT36CKvDb3+aBEFg=
-golang.org/x/sys v0.30.0 h1:QjkSwP/36a20jFYWkSue1YwXzLmsV5Gfq7Eiy72C1uc=
-golang.org/x/sys v0.30.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
-golang.org/x/text v0.28.0 h1:rhazDwis8INMIwQ4tpjLDzUhx6RlXqZNPEM0huQojng=
-golang.org/x/text v0.28.0/go.mod h1:U8nCwOR8jO/marOQ0QbDiOngZVEBB7MAiitBuMjXiNU=
+golang.org/x/sys v0.47.0 h1:o7XGOvZQCADBQQ4Y7VNq2dRWQR7JmOUW8Kxx4ZsNgWs=
+golang.org/x/sys v0.47.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=
+golang.org/x/text v0.40.0 h1:Ub2Z6/xjgF1WrYQz2nuITOEegKFtiIy+rieRJ5lHZKs=
+golang.org/x/text v0.40.0/go.mod h1:hpnzDAfGV753zIKo+wk3u1bVKCGPbrnF7+7LBF/UHVY=
+google.golang.org/protobuf v1.36.11 h1:fV6ZwhNocDyBLK0dj+fg8ektcVegBBuEolpbTQyBNVE=
+google.golang.org/protobuf v1.36.11/go.mod h1:HTf+CrKn2C3g5S8VImy6tdcUvCska2kB7j23XfzDpco=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/check.v1 v1.0.0-20190902080502-41f04d3bba15 h1:YR8cESwS4TdDjEe65xsg0ogRM/Nc3DYOhEAlW+xobZo=
 gopkg.in/check.v1 v1.0.0-20190902080502-41f04d3bba15/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
@@ -44,6 +44,15 @@ var (
 	errNotPort = errors.New("must be a port number, 1 to 65535")
 	errNotBool = errors.New("must be true or false")
 	errNotIP   = errors.New("must be an IP address, or empty")
+
+	errMetricsCredentials = errors.New(
+		"METRICS_USERNAME and METRICS_PASSWORD must be set together, " +
+			"or neither",
+	)
+	errMetricsUsernameColon = errors.New(
+		"METRICS_USERNAME must not contain \":\", " +
+			"which basic auth cannot carry in a user name",
+	)
 )

 // Params defines the dependencies for Config.
@@ -178,6 +187,18 @@ func (s *Config) check() error {
 		}
 	}

+	// The server records and serves metrics only with both set, so
+	// one alone is a mistake that would otherwise go unnoticed.
+	if (s.MetricsUsername == "") != (s.MetricsPassword == "") {
+		return errMetricsCredentials
+	}
+
+	// Basic auth splits the credentials at the first ":", so with one
+	// in the user name every request to /metrics would get 401.
+	if strings.Contains(s.MetricsUsername, ":") {
+		return errMetricsUsernameColon
+	}
+
 	return checkOrigins(s.CORSAllowedOrigins)
 }

@@ -103,6 +103,32 @@ func TestDataDirMaxBytesMustBeANumber(t *testing.T) {
 	requireConfigError(t, "DATA_DIR_MAX_BYTES")
 }

+// TestMetricsCredentialsGoTogether: with only one of the two set, the
+// server would quietly serve no metrics, so the start fails, naming
+// both.
+func TestMetricsCredentialsGoTogether(t *testing.T) {
+	for _, set := range []string{"METRICS_USERNAME", "METRICS_PASSWORD"} {
+		t.Run(set, func(t *testing.T) {
+			t.Setenv("METRICS_USERNAME", "")
+			t.Setenv("METRICS_PASSWORD", "")
+			t.Setenv(set, "prometheus")
+
+			requireConfigError(t, "METRICS_USERNAME")
+			requireConfigError(t, "METRICS_PASSWORD")
+		})
+	}
+}
+
+// TestMetricsUsernameMustNotContainColon: basic auth splits the
+// credentials at the first ":", so such a user name would get 401 on
+// every request to /metrics.
+func TestMetricsUsernameMustNotContainColon(t *testing.T) {
+	t.Setenv("METRICS_USERNAME", "prom:etheus")
+	t.Setenv("METRICS_PASSWORD", "secret")
+
+	requireConfigError(t, "METRICS_USERNAME")
+}
+
 // TestCORSAllowedOriginsMustBeOrigins: "*" would let every origin in,
 // and an entry that is not a plain origin would match no page.
 func TestCORSAllowedOriginsMustBeOrigins(t *testing.T) {
@@ -18,9 +18,14 @@ import (
 	"sneak.berlin/go/netwatch/internal/globals"
 	"sneak.berlin/go/netwatch/internal/logger"

+	basicauth "github.com/99designs/basicauth-go"
 	"github.com/go-chi/chi/v5/middleware"
 	"github.com/go-chi/cors"
 	"github.com/go-chi/httprate"
+	"github.com/prometheus/client_golang/prometheus"
+	metrics "github.com/slok/go-http-metrics/metrics/prometheus"
+	ghmm "github.com/slok/go-http-metrics/middleware"
+	"github.com/slok/go-http-metrics/middleware/std"
 	"go.uber.org/fx"
 )

@@ -364,3 +369,25 @@ func (s *Middleware) RateLimit(
 		),
 	)
 }
+
+// Metrics returns middleware that records each request's duration and
+// response size, and the requests in progress, in registry. They are
+// labelled by the request path.
+func (s *Middleware) Metrics(
+	registry prometheus.Registerer,
+) func(http.Handler) http.Handler {
+	mdlw := ghmm.New(ghmm.Config{
+		Recorder: metrics.NewRecorder(metrics.Config{Registry: registry}),
+	})
+
+	return std.HandlerProvider("", mdlw)
+}
+
+// MetricsAuth returns middleware that lets a request through only with
+// METRICS_USERNAME and METRICS_PASSWORD as its basic auth credentials,
+// and answers any other with 401.
+func (s *Middleware) MetricsAuth() func(http.Handler) http.Handler {
+	return basicauth.New("metrics", map[string][]string{
+		s.params.Config.MetricsUsername: {s.params.Config.MetricsPassword},
+	})
+}
@@ -5,6 +5,9 @@ import (

 	"github.com/go-chi/chi/v5"
 	"github.com/go-chi/chi/v5/middleware"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/collectors"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
 )

 const (
@@ -29,13 +32,37 @@ func (s *Server) SetupRoutes() {
 	s.router.Use(s.mw.MaxBodyBytes(maxRequestBodyBytes))
 	s.router.Use(middleware.Timeout(requestTimeout))

-	s.router.Get(
-		"/.well-known/healthcheck",
-		s.h.HandleHealthCheck(),
+	// The metrics go in a registry of this server's own, not in
+	// Prometheus' default one, which takes them only once per process.
+	registry := prometheus.NewRegistry()
+	registry.MustRegister(
+		collectors.NewGoCollector(),
+		collectors.NewProcessCollector(collectors.ProcessCollectorOpts{}),
 	)

-	s.router.Route("/api/v1", func(r chi.Router) {
+	// Requests are measured only once chi has matched them to one of
+	// these routes, by path and method. The metrics are labelled with
+	// both, which any client can make up, so measuring every request
+	// would let clients add labels without bound. A Route here would
+	// be matched by its path prefix alone, so each path is given in
+	// full.
+	s.router.Group(func(r chi.Router) {
+		// config.New refuses one of the two credentials without the
+		// other.
+		if s.params.Config.MetricsUsername != "" {
+			r.Use(s.mw.Metrics(registry))
+		}
+
+		r.Get("/.well-known/healthcheck", s.h.HandleHealthCheck())
+
 		r.With(s.mw.RateLimit(s.params.Config.ReportsPerMinute)).
-			Post("/reports", s.h.HandleReport())
+			Post("/api/v1/reports", s.h.HandleReport())
 	})
+
+	if s.params.Config.MetricsUsername != "" {
+		s.router.With(s.mw.MetricsAuth()).
+			Get("/metrics", promhttp.HandlerFor(
+				registry, promhttp.HandlerOpts{},
+			).ServeHTTP)
+	}
 }
@@ -107,6 +107,95 @@ func TestCORSAllowedOriginsReachTheRouter(t *testing.T) {
 	}
 }

+// TestNoMetricsWithoutCredentials: with neither metrics setting set,
+// there is no /metrics.
+func TestNoMetricsWithoutCredentials(t *testing.T) {
+	t.Setenv("METRICS_USERNAME", "")
+	t.Setenv("METRICS_PASSWORD", "")
+
+	srv := newServer(t)
+	srv.SetupRoutes()
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(t.Context(),
+		http.MethodGet, "/metrics", http.NoBody)
+	srv.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusNotFound {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusNotFound)
+	}
+}
+
+// TestMetricsBehindBasicAuth: with both metrics settings set, /metrics
+// answers only with them as basic auth credentials, and shows a
+// request to a route but not one to a path no route has.
+func TestMetricsBehindBasicAuth(t *testing.T) {
+	t.Setenv("METRICS_USERNAME", "prometheus")
+	t.Setenv("METRICS_PASSWORD", "right")
+
+	srv := newServer(t)
+	srv.SetupRoutes()
+
+	get := func(path, username, password string) *httptest.ResponseRecorder {
+		rec := httptest.NewRecorder()
+		req := httptest.NewRequestWithContext(t.Context(),
+			http.MethodGet, path, http.NoBody)
+
+		if username != "" {
+			req.SetBasicAuth(username, password)
+		}
+
+		srv.ServeHTTP(rec, req)
+
+		return rec
+	}
+
+	get("/.well-known/healthcheck", "", "")
+	get("/api/v1/no-such-route", "", "")
+
+	for _, creds := range [][2]string{
+		{"", ""},
+		{"prometheus", "wrong"},
+		{"someone", "right"},
+	} {
+		rec := get("/metrics", creds[0], creds[1])
+		if rec.Code != http.StatusUnauthorized {
+			t.Errorf("credentials %q: status = %d, want %d",
+				creds, rec.Code, http.StatusUnauthorized)
+		}
+	}
+
+	rec := get("/metrics", "prometheus", "right")
+	if rec.Code != http.StatusOK {
+		t.Fatalf("right credentials: status = %d, want %d",
+			rec.Code, http.StatusOK)
+	}
+
+	body := rec.Body.String()
+	if !strings.Contains(body, `handler="/.well-known/healthcheck"`) {
+		t.Errorf("metrics show no health check request:\n%s", body)
+	}
+
+	if strings.Contains(body, "no-such-route") {
+		t.Errorf("metrics show a request to a path no route has:\n%s", body)
+	}
+
+	if !strings.Contains(body, "go_goroutines") {
+		t.Errorf("metrics show no Go runtime metrics:\n%s", body)
+	}
+}
+
+// TestMetricsInTwoServers: two servers in one process can both have
+// metrics on.
+func TestMetricsInTwoServers(t *testing.T) {
+	t.Setenv("METRICS_USERNAME", "prometheus")
+	t.Setenv("METRICS_PASSWORD", "right")
+
+	for range 2 {
+		newServer(t).SetupRoutes()
+	}
+}
+
 // TestHealthCheckRejectsOversizeBody sends the health check, which
 // never reads its body, a body one byte over the limit. Only the
 // router-wide body limit can reject it.