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;
NewForTest gives its Middleware a recorder on a fresh registry. The
scrape keeps the same series and labels, including go_*, process_*
and promhttp_metric_handler_*.

Model: opus-5-5

internal/delivery/export_test.go

@@ -9,6 +9,7 @@ import (
 	"net/url"
 	"time"
 
+	"github.com/prometheus/client_golang/prometheus"
 	"go.uber.org/fx"
 	"gorm.io/gorm"
 	"sneak.berlin/go/webhooker/internal/database"
@@ -389,7 +390,7 @@ func NewTestEngine(
 		deliveryCh: make(chan Task, deliveryChannelSize),
 		retryCh:    make(chan Task, retryChannelSize),
 		workers:    workers,
-		mtr:        metrics.Default(),
+		mtr:        metrics.New(prometheus.NewRegistry()),
 	}
 	e.initTargets(client)
 
@@ -404,7 +405,7 @@ func NewTestEngineSmallRetry(
 	e := &Engine{
 		log:     log,
 		retryCh: make(chan Task, 1),
-		mtr:     metrics.Default(),
+		mtr:     metrics.New(prometheus.NewRegistry()),
 	}
 	e.initTargets(nil)
 
@@ -427,7 +428,7 @@ func NewTestEngineWithDB(
 		deliveryCh: make(chan Task, deliveryChannelSize),
 		retryCh:    make(chan Task, retryChannelSize),
 		workers:    workers,
-		mtr:        metrics.Default(),
+		mtr:        metrics.New(prometheus.NewRegistry()),
 	}
 	e.initTargets(client)
 
@@ -435,8 +436,7 @@ func NewTestEngineWithDB(
 }
 
 // ExportSetMetrics substitutes the engine's metric set, so a test can
-// assert on collectors registered on a private registry instead of
-// the process-wide ones every other test is also moving.
+// assert on collectors registered on a registry it holds.
 func (e *Engine) ExportSetMetrics(mtr *metrics.Set) {
 	e.mtr = mtr
 }