[#164] metrics: Fill local registry explicitly

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

pkg/metrics/registry.go

@@ -0,0 +1,40 @@
+package metrics
+
+import (
+	"net/http"
+	"sync"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/collectors"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+// Handler returns an http.Handler for the local registry.
+func Handler() http.Handler {
+	promhttp.Handler()
+	return promhttp.InstrumentMetricHandler(
+		registry,
+		promhttp.HandlerFor(registry, promhttp.HandlerOpts{}))
+}
+
+var (
+	registry = prometheus.NewRegistry()
+	// registeredCollectorsMtx protects collectors slice.
+	// It should not be acessed concurrently, but we can easily forget this in future, thus this mutex.
+	registeredCollectorsMtx sync.Mutex
+	registeredCollectors    []prometheus.Collector
+)
+
+func init() {
+	mustRegister(collectors.NewProcessCollector(collectors.ProcessCollectorOpts{}))
+	mustRegister(collectors.NewGoCollector())
+}
+
+func mustRegister(cs ...prometheus.Collector) {
+	for i := range cs {
+		registry.MustRegister(cs[i])
+	}
+	registeredCollectorsMtx.Lock()
+	registeredCollectors = append(registeredCollectors, cs...)
+	registeredCollectorsMtx.Unlock()
+}