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

The HTTP metrics recorder, the delivery collectors and the Go and
process collectors now register on one prometheus.Registry that fx
provides, instead of Prometheus's global default registry, and
/metrics serves that registry. A second metrics-enabled router in one
process, or the server tests run with -count=2, no longer panics on a
duplicate registration.

The middleware builds its recorder once, in New, so installing
Metrics() on more than one router over the same graph is also safe.
The scrape keeps the same series and labels, including go_*,
process_* and promhttp_metric_handler_*.

Model: opus-5-5

internal/metrics/metrics.go

@@ -3,17 +3,17 @@
 // deliveries are attempted, how they end, how long they take, how
 // deep the queues are, and how many circuit breakers are open.
 //
-// The inbound HTTP metrics come from the go-http-metrics recorder in
-// internal/middleware and land on prometheus.DefaultRegisterer. These
-// collectors register there too, so both surfaces are gathered by the
-// one promhttp handler mounted on the authenticated /metrics route.
+// It also builds the registry the authenticated /metrics route
+// serves. These collectors, the inbound HTTP metrics recorded in
+// internal/middleware, and the Go runtime and process collectors all
+// register on that one registry, never on Prometheus's global default.
 package metrics
 
 import (
-	"sync"
 	"time"
 
 	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/collectors"
 	"github.com/prometheus/client_golang/prometheus/promauto"
 	"sneak.berlin/go/webhooker/internal/database"
 )
@@ -57,25 +57,31 @@ var knownTargetTypes = []database.TargetType{
 	database.TargetTypeSlack,
 }
 
-// defaultSet is the process-wide metric set, registered on the same
-// registry the HTTP middleware and the /metrics handler already use.
-// It is built on first use rather than in an init so that a test
-// binary that never touches metrics never registers them.
+// NewRegistry returns the registry /metrics serves, carrying the Go
+// runtime and process collectors that Prometheus's global default
+// registry carries, so the go_* and process_* series stay in the
+// scrape.
 //
-//nolint:gochecknoglobals // one process-wide registration, by design
-var defaultSet = sync.OnceValue(func() *Set {
-	return New(prometheus.DefaultRegisterer)
-})
+// A registry of its own, rather than the global default, is what lets
+// two dependency graphs in one process — two tests, say — each
+// register their collectors without the second registration
+// panicking.
+func NewRegistry() *prometheus.Registry {
+	reg := prometheus.NewRegistry()
+	reg.MustRegister(
+		collectors.NewGoCollector(),
+		collectors.NewProcessCollector(
+			collectors.ProcessCollectorOpts{},
+		),
+	)
 
-// Default returns the process-wide metric set.
-func Default() *Set {
-	return defaultSet()
+	return reg
 }
 
 // Set is one registered group of webhooker's delivery collectors.
-// Production uses the single Default set; tests build their own
-// against a private registry so assertions are not disturbed by
-// deliveries other tests are making concurrently.
+// Production builds one on the registry /metrics serves; tests build
+// their own against a private registry so assertions are not
+// disturbed by deliveries other tests are making concurrently.
 type Set struct {
 	eventsReceived      prometheus.Counter
 	deliveryAttempts    *prometheus.CounterVec
@@ -93,7 +99,7 @@ type Set struct {
 // New registers a full set of delivery collectors on reg and returns
 // it. It panics if reg already holds them, which is the intended
 // behaviour for a duplicate registration.
-func New(reg prometheus.Registerer) *Set {
+func New(reg *prometheus.Registry) *Set {
 	factory := promauto.With(reg)
 
 	s := &Set{