refactor: default to prometheus.DefaultRegisterer (#722)

- removes calls to prometheus.NewRegistry()
- replaces NewRegistry() call with global `prometheus.DefaultRegisterer`
  so by default boxo users who did not specify custom registry
  are not missing any metrics.
- ensures we don't panic if tests run in parallel and DefaultRegisterer is used

CHANGELOG.md

@@ -49,7 +49,7 @@ The following emojis are used to highlight certain changes:
 
   - The above is only necessary if content routing is needed. Otherwise:
 
-```
+```go
 	// Create network: no contentRouter anymore
 	bswapnet := network.NewFromIpfsHost(host)
 	// Create Bitswap: a new "discovery" parameter set to nil (disable content discovery)
@@ -64,6 +64,8 @@ The following emojis are used to highlight certain changes:
 
 - `routing/http/server`: added built-in Prometheus instrumentation to http delegated `/routing/v1/` endpoints, with custom buckets for response size and duration to match real world data observed at [the `delegated-ipfs.dev` instance](https://docs.ipfs.tech/concepts/public-utilities/#delegated-routing). [#718](https://github.com/ipfs/boxo/pull/718) [#724](https://github.com/ipfs/boxo/pull/724)
 - `routing/http/server`: added configurable routing timeout (`DefaultRoutingTimeout` being 30s) to prevent indefinite hangs during content/peer routing. Set custom duration via `WithRoutingTimeout`. [#720](https://github.com/ipfs/boxo/pull/720)
+- `routing/http/server`: exposes Prometheus metrics on `prometheus.DefaultRegisterer` and a custom one can be provided via `WithPrometheusRegistry` [#722](https://github.com/ipfs/boxo/pull/722)
+- `gateway`: `NewCacheBlockStore` and `NewCarBackend` will use `prometheus.DefaultRegisterer` when a custom one is not specified via `WithPrometheusRegistry` [#722](https://github.com/ipfs/boxo/pull/722)
 
 ### Changed
 

gateway/backend_car.go

@@ -73,7 +73,7 @@ func NewCarBackend(f CarFetcher, opts ...BackendOption) (*CarBackend, error) {
 		return nil, err
 	}
 
-	var promReg prometheus.Registerer = prometheus.NewRegistry()
+	var promReg prometheus.Registerer = prometheus.DefaultRegisterer
 	if compiledOptions.promRegistry != nil {
 		promReg = compiledOptions.promRegistry
 	}
@@ -117,6 +117,11 @@ func NewRemoteCarBackend(gatewayURL []string, httpClient *http.Client, opts ...B
 }
 
 func registerCarBackendMetrics(promReg prometheus.Registerer) *CarBackendMetrics {
+	// make sure we have functional registry
+	if promReg == nil {
+		promReg = prometheus.DefaultRegisterer
+	}
+
 	// How many CAR Fetch attempts we had? Need this to calculate % of various car request types.
 	// We only count attempts here, because success/failure with/without retries are provided by caboose:
 	// - ipfs_caboose_fetch_duration_car_success_count
@@ -129,15 +134,15 @@ func registerCarBackendMetrics(promReg prometheus.Registerer) *CarBackendMetrics
 		Name:      "car_fetch_attempts",
 		Help:      "The number of times a CAR fetch was attempted by IPFSBackend.",
 	})
-	promReg.MustRegister(carFetchAttemptMetric)
+	registerMetric(promReg, carFetchAttemptMetric)
 
 	contextAlreadyCancelledMetric := prometheus.NewCounter(prometheus.CounterOpts{
 		Namespace: "ipfs",
 		Subsystem: "gw_car_backend",
 		Name:      "car_fetch_context_already_cancelled",
 		Help:      "The number of times context is already cancelled when a CAR fetch was attempted by IPFSBackend.",
 	})
-	promReg.MustRegister(contextAlreadyCancelledMetric)
+	registerMetric(promReg, contextAlreadyCancelledMetric)
 
 	// How many blocks were read via CARs?
 	// Need this as a baseline to reason about error ratio vs raw_block_recovery_attempts.
@@ -147,15 +152,15 @@ func registerCarBackendMetrics(promReg prometheus.Registerer) *CarBackendMetrics
 		Name:      "car_blocks_fetched",
 		Help:      "The number of blocks successfully read via CAR fetch.",
 	})
-	promReg.MustRegister(carBlocksFetchedMetric)
+	registerMetric(promReg, carBlocksFetchedMetric)
 
 	carParamsMetric := prometheus.NewCounterVec(prometheus.CounterOpts{
 		Namespace: "ipfs",
 		Subsystem: "gw_car_backend",
 		Name:      "car_fetch_params",
 		Help:      "How many times specific CAR parameter was used during CAR data fetch.",
 	}, []string{"dagScope", "entityRanges"}) // we use 'ranges' instead of 'bytes' here because we only count the number of ranges present
-	promReg.MustRegister(carParamsMetric)
+	registerMetric(promReg, carParamsMetric)
 
 	bytesRangeStartMetric := prometheus.NewHistogram(prometheus.HistogramOpts{
 		Namespace: "ipfs",
@@ -164,7 +169,7 @@ func registerCarBackendMetrics(promReg prometheus.Registerer) *CarBackendMetrics
 		Help:      "Tracks where did the range request start.",
 		Buckets:   prometheus.ExponentialBuckets(1024, 2, 24), // 1024 bytes to 8 GiB
 	})
-	promReg.MustRegister(bytesRangeStartMetric)
+	registerMetric(promReg, bytesRangeStartMetric)
 
 	bytesRangeSizeMetric := prometheus.NewHistogram(prometheus.HistogramOpts{
 		Namespace: "ipfs",
@@ -173,7 +178,7 @@ func registerCarBackendMetrics(promReg prometheus.Registerer) *CarBackendMetrics
 		Help:      "Tracks the size of range requests.",
 		Buckets:   prometheus.ExponentialBuckets(256*1024, 2, 10), // From 256KiB to 100MiB
 	})
-	promReg.MustRegister(bytesRangeSizeMetric)
+	registerMetric(promReg, bytesRangeSizeMetric)
 
 	return &CarBackendMetrics{
 		contextAlreadyCancelledMetric,

gateway/blockstore.go

@@ -35,16 +35,15 @@ var _ blockstore.Blockstore = (*cacheBlockStore)(nil)
 // in memory using a two queue cache. It can be useful, for example, when paired
 // with a proxy blockstore (see [NewRemoteBlockstore]).
 //
-// If the given [prometheus.Registerer] is nil, a new one will be created using
-// [prometheus.NewRegistry].
+// If the given [prometheus.Registerer] is nil, a [prometheus.DefaultRegisterer] will be used.
 func NewCacheBlockStore(size int, reg prometheus.Registerer) (blockstore.Blockstore, error) {
 	c, err := lru.New2Q[string, []byte](size)
 	if err != nil {
 		return nil, err
 	}
 
 	if reg == nil {
-		reg = prometheus.NewRegistry()
+		reg = prometheus.DefaultRegisterer
 	}
 
 	cacheHitsMetric := prometheus.NewCounter(prometheus.CounterOpts{
@@ -61,15 +60,8 @@ func NewCacheBlockStore(size int, reg prometheus.Registerer) (blockstore.Blockst
 		Help:      "The number of global block cache requests.",
 	})
 
-	err = reg.Register(cacheHitsMetric)
-	if err != nil {
-		return nil, err
-	}
-
-	err = reg.Register(cacheRequestsMetric)
-	if err != nil {
-		return nil, err
-	}
+	registerMetric(reg, cacheHitsMetric)
+	registerMetric(reg, cacheRequestsMetric)
 
 	return &cacheBlockStore{
 		cache:               c,

gateway/metrics.go

@@ -307,3 +307,14 @@ var tracer = otel.Tracer("boxo/gateway")
 func spanTrace(ctx context.Context, spanName string, opts ...trace.SpanStartOption) (context.Context, trace.Span) {
 	return tracer.Start(ctx, "Gateway."+spanName, opts...)
 }
+
+// registerMetric registers metrics in registry or logs an error.
+//
+// Registration may error if metric is alreadyregistered. we are not using
+// MustRegister here to allow people to run tests in parallel without having to
+// write tedious  glue code that creates unique registry for each unit test
+func registerMetric(registry prometheus.Registerer, metric prometheus.Collector) {
+	if err := registry.Register(metric); err != nil {
+		log.Errorf("failed to register %v: %v", metric, err)
+	}
+}

routing/http/server/server.go

@@ -10,6 +10,7 @@ import (
 	"mime"
 	"net/http"
 	"strings"
+	"sync/atomic"
 	"time"
 
 	"github.com/cespare/xxhash/v2"
@@ -152,14 +153,25 @@ func Handler(svc ContentRouter, opts ...Option) http.Handler {
 	}
 
 	if server.promRegistry == nil {
-		server.promRegistry = prometheus.NewRegistry()
+		server.promRegistry = prometheus.DefaultRegisterer
+	}
+
+	// Workaround due to https://github.com/slok/go-http-metrics
+	// using egistry.MustRegister internally.
+	// In production there will be only one handler, however we append counter
+	// to ensure duplicate metric registration will not panic in parallel tests
+	// when global prometheus.DefaultRegisterer is used.
+	metricsPrefix := "delegated_routing_server"
+	c := handlerCount.Add(1)
+	if c > 1 {
+		metricsPrefix = fmt.Sprintf("%s_%d", metricsPrefix, c)
 	}
 
 	// Create middleware with prometheus recorder
 	mdlw := middleware.New(middleware.Config{
 		Recorder: metrics.NewRecorder(metrics.Config{
 			Registry:        server.promRegistry,
-			Prefix:          "delegated_routing_server",
+			Prefix:          metricsPrefix,
 			SizeBuckets:     prometheus.ExponentialBuckets(100, 4, 8), // [100 400 1600 6400 25600 102400 409600 1.6384e+06]
 			DurationBuckets: []float64{0.1, 0.5, 1, 2, 5, 8, 10, 20, 30},
 		}),
@@ -176,6 +188,8 @@ func Handler(svc ContentRouter, opts ...Option) http.Handler {
 	return r
 }
 
+var handlerCount atomic.Int32
+
 type server struct {
 	svc                   ContentRouter
 	disableNDJSON         bool