Export Prometheus via OpenCensus and register the DHT metrics

This uses OC to export metrics via Prometheus. This also means that other OC metrics, such as the ones in go-libp2p-kad-dht can be exposed (via Prometheus, and whatever other exporters are used). I did some checks that no existing metrics move as a result.

core/corehttp/metrics.go

@@ -6,14 +6,27 @@ import (
 
 	core "github.com/ipfs/go-ipfs/core"
 
+	dhtMetrics "github.com/libp2p/go-libp2p-kad-dht/metrics"
 	prometheus "github.com/prometheus/client_golang/prometheus"
 	promhttp "github.com/prometheus/client_golang/prometheus/promhttp"
+	ocProm "go.opencensus.io/exporter/prometheus"
+	"go.opencensus.io/stats/view"
 )
 
 // This adds the scraping endpoint which Prometheus uses to fetch metrics.
 func MetricsScrapingOption(path string) ServeOption {
 	return func(n *core.IpfsNode, _ net.Listener, mux *http.ServeMux) (*http.ServeMux, error) {
-		mux.Handle(path, promhttp.HandlerFor(prometheus.DefaultGatherer, promhttp.HandlerOpts{}))
+		pe, err := ocProm.NewExporter(ocProm.Options{
+			Registry: prometheus.DefaultGatherer.(*prometheus.Registry),
+		})
+		if err != nil {
+			return mux, err
+		}
+		view.RegisterExporter(pe)
+		if err := view.Register(dhtMetrics.Views...); err != nil {
+			return mux, err
+		}
+		mux.Handle(path, pe)
 		return mux, nil
 	}
 }

go.mod

@@ -70,7 +70,7 @@ require (
 	github.com/libp2p/go-libp2p-crypto v0.0.1
 	github.com/libp2p/go-libp2p-host v0.0.1
 	github.com/libp2p/go-libp2p-interface-connmgr v0.0.1
-	github.com/libp2p/go-libp2p-kad-dht v0.0.9
+	github.com/libp2p/go-libp2p-kad-dht v0.0.10-0.20190423053654-33dc9eda5233
 	github.com/libp2p/go-libp2p-kbucket v0.1.1
 	github.com/libp2p/go-libp2p-loggables v0.0.1
 	github.com/libp2p/go-libp2p-metrics v0.0.1
@@ -110,6 +110,7 @@ require (
 	github.com/whyrusleeping/go-sysinfo v0.0.0-20190219211824-4a357d4b90b1
 	github.com/whyrusleeping/multiaddr-filter v0.0.0-20160516205228-e903e4adabd7
 	github.com/whyrusleeping/tar-utils v0.0.0-20180509141711-8c6c8ba81d5c
+	go.opencensus.io v0.20.2
 	go.uber.org/atomic v1.3.2 // indirect
 	go.uber.org/dig v1.7.0 // indirect
 	go.uber.org/fx v1.9.0

go.sum

@@ -343,8 +343,8 @@ github.com/libp2p/go-libp2p-interface-connmgr v0.0.1/go.mod h1:GarlRLH0LdeWcLnYM
 github.com/libp2p/go-libp2p-interface-pnet v0.0.1 h1:7GnzRrBTJHEsofi1ahFdPN9Si6skwXQE9UqR2S+Pkh8=
 github.com/libp2p/go-libp2p-interface-pnet v0.0.1/go.mod h1:el9jHpQAXK5dnTpKA4yfCNBZXvrzdOU75zz+C6ryp3k=
 github.com/libp2p/go-libp2p-kad-dht v0.0.4/go.mod h1:oaBflOQcuC8H+SVV0YN26H6AS+wcUEJyjUGV66vXuSY=
-github.com/libp2p/go-libp2p-kad-dht v0.0.9 h1:lgt0TuhnZieDg7jd6e4+aZIL11+Z9MLz7h66duB04Hw=
-github.com/libp2p/go-libp2p-kad-dht v0.0.9/go.mod h1:RVllrB76xoaayqqBkLmXT8iIkMRNfLWSBuqAggP7W00=
+github.com/libp2p/go-libp2p-kad-dht v0.0.10-0.20190423053654-33dc9eda5233 h1:XFz2azHEr07wTbCuDxgdOUVAIfEV17zqfLe+uN+yU8M=
+github.com/libp2p/go-libp2p-kad-dht v0.0.10-0.20190423053654-33dc9eda5233/go.mod h1:RVllrB76xoaayqqBkLmXT8iIkMRNfLWSBuqAggP7W00=
 github.com/libp2p/go-libp2p-kbucket v0.0.1 h1:7H5hM851hkEpLOFjrVNSrrxo6J4bWrUQxxv+z1JW9xk=
 github.com/libp2p/go-libp2p-kbucket v0.0.1/go.mod h1:Y0iQDHRTk/ZgM8PC4jExoF+E4j+yXWwRkdldkMa5Xm4=
 github.com/libp2p/go-libp2p-kbucket v0.1.1 h1:ZrvW3qCM+lAuv7nrNts/zfEiClq+GZe8OIzX4Vb3Dwo=