add opentracing spans for metrics

vendor/github.com/rcrowley/go-metrics/graphite_test.go

@@ -0,0 +1,22 @@
+package metrics
+
+import (
+	"net"
+	"time"
+)
+
+func ExampleGraphite() {
+	addr, _ := net.ResolveTCPAddr("net", ":2003")
+	go Graphite(DefaultRegistry, 1*time.Second, "some.prefix", addr)
+}
+
+func ExampleGraphiteWithConfig() {
+	addr, _ := net.ResolveTCPAddr("net", ":2003")
+	go GraphiteWithConfig(GraphiteConfig{
+		Addr:          addr,
+		Registry:      DefaultRegistry,
+		FlushInterval: 1 * time.Second,
+		DurationUnit:  time.Millisecond,
+		Percentiles:   []float64{0.5, 0.75, 0.99, 0.999},
+	})
+}