Api metrics (#1014)

* api metrics support

* comments reflected

* metrics middleware fix

api/server/stats.go

@@ -0,0 +1,68 @@
+package server
+
+import (
+	"github.com/sirupsen/logrus"
+	"go.opencensus.io/stats"
+	"go.opencensus.io/stats/view"
+	"go.opencensus.io/tag"
+)
+
+var (
+	apiRequestCount = stats.Int64("api/request_count", "Number of API requests", stats.UnitDimensionless)
+	apiLatency      = stats.Float64("api/latency", "API latency", stats.UnitMilliseconds)
+)
+
+var (
+	pathKey   = makeKey("path")
+	methodKey = makeKey("method")
+	statusKey = makeKey("status")
+)
+
+var (
+	defaultLatencyDistribution = view.Distribution(0, 1, 2, 3, 4, 5, 6, 8, 10, 13, 16, 20, 25, 30, 40, 50, 65, 80, 100, 130, 160, 200, 250, 300, 400, 500, 650, 800, 1000, 2000, 5000, 10000, 20000, 50000, 100000)
+)
+
+var (
+	ApiRequestCountView = &view.View{
+		Name:        "api/request_count",
+		Description: "Count of API requests started",
+		Measure:     apiRequestCount,
+		TagKeys:     []tag.Key{pathKey, methodKey},
+		Aggregation: view.Count(),
+	}
+
+	ApiResponseCountView = &view.View{
+		Name:        "api/response_count",
+		Description: "API response count",
+		TagKeys:     []tag.Key{pathKey, methodKey, statusKey},
+		Measure:     apiLatency,
+		Aggregation: view.Count(),
+	}
+
+	ApiLatencyView = &view.View{
+		Name:        "api/latency",
+		Description: "Latency distribution of API requests",
+		Measure:     apiLatency,
+		TagKeys:     []tag.Key{pathKey, methodKey, statusKey},
+		Aggregation: defaultLatencyDistribution,
+	}
+)
+
+func registerViews() {
+	err := view.Register(
+		ApiRequestCountView,
+		ApiResponseCountView,
+		ApiLatencyView,
+	)
+	if err != nil {
+		logrus.WithError(err).Fatal("cannot register view")
+	}
+}
+
+func makeKey(name string) tag.Key {
+	key, err := tag.NewKey(name)
+	if err != nil {
+		logrus.Fatal(err)
+	}
+	return key
+}