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

backend/go.mod

@@ -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
 )