Serve /metrics from a registry of its own (closes #227)

A second metrics-enabled router in one process panicked on a duplicate collector registration, because every collector registered on Prometheus's global default registry. metrics.NewRegistry now builds one registry with the Go runtime and process collectors; fx provides it and the delivery metric set built on it. The middleware builds its HTTP recorder once on that registry (NewForTest on a fresh one), the engine and handlers take the metric set from fx, and nothing registers on the global default any more.

/metrics is served from the new registry with the same series names, labels and auth. A test builds two metrics-enabled routers in one process.

Model: opus-5-5

internal/middleware/testing.go

@@ -3,12 +3,17 @@ package middleware
 import (
 	"log/slog"
 
+	"github.com/prometheus/client_golang/prometheus"
+	prommetrics "github.com/slok/go-http-metrics/metrics/prometheus"
 	"sneak.berlin/go/webhooker/internal/config"
 	"sneak.berlin/go/webhooker/internal/session"
 )
 
 // NewForTest creates a Middleware with the minimum dependencies
 // needed for testing. This bypasses the fx lifecycle.
+//
+// Its metrics recorder writes to a fresh registry of its own, so
+// Metrics() works on it and two of them never collide.
 func NewForTest(
 	log *slog.Logger,
 	cfg *config.Config,
@@ -20,5 +25,8 @@ func NewForTest(
 			Config: cfg,
 		},
 		session: sess,
+		metricsRecorder: prommetrics.NewRecorder(
+			prommetrics.Config{Registry: prometheus.NewRegistry()},
+		),
 	}
 }