Add prometheus metrics collectors for http handlers (#958)

* Add prometheus metrics exporter for http handlers

* Use prometheus from opencensus

* Add comment to unused func() param

* Add package names to operation description in observ package

* Sort exported/unexported functions

pkg/config/config_test.go

@@ -222,6 +222,7 @@ func TestParseExampleConfig(t *testing.T) {
 		Storage:          expStorage,
 		TraceExporterURL: "http://localhost:14268",
 		TraceExporter:    "jaeger",
+		StatsExporter:    "prometheus",
 	}
 
 	absPath, err := filepath.Abs(testConfigFile(t))