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 and serves them at
GET /metrics behind basic auth with those credentials; nginx passes
/metrics to it. With neither set there is no such route; one alone
stops the start with an error naming both.

Only requests chi has matched to a route are recorded, not every
request as the conventions show: the labels are path and method, which
clients can make up without end. So POST /api/v1/reports is registered
by its full path, not inside a route group.

Deviation: go get and go mod tidy ran directly; no entrypoint adds a Go
dependency yet (#45).

Model: opus-5-5

backend/internal/middleware/middleware.go

@@ -18,9 +18,13 @@ 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"
+	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 +368,25 @@ func (s *Middleware) RateLimit(
 		),
 	)
 }
+
+// Metrics returns middleware that records each request's duration and
+// response size, and the requests in progress, in Prometheus' default
+// registry, which GET /metrics serves. They are labelled by the request
+// path. Call it once per process: the registry refuses the same metrics
+// twice, and this panics.
+func (s *Middleware) Metrics() func(http.Handler) http.Handler {
+	mdlw := ghmm.New(ghmm.Config{
+		Recorder: metrics.NewRecorder(metrics.Config{}),
+	})
+
+	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},
+	})
+}