prometheus: add status metrics

Add a helper function to register the same metrics
for both the worker and composer - the only difference
being the subsystem name. The function checks if the
metric has already been registered and, if so, returns
the already registered metric.

internal/prometheus/status_metrics.go

@@ -0,0 +1,30 @@
+package prometheus
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+func StatusRequestsCounter(subsystem string) *prometheus.CounterVec {
+	// return a function so we can use this for both
+	// composer & worker metrics
+	reg := prometheus.NewRegistry()
+	counter := promauto.With(reg).NewCounterVec(prometheus.CounterOpts{
+		Name:      "request_count",
+		Namespace: Namespace,
+		Subsystem: subsystem,
+		Help:      "total number of http requests",
+	}, []string{"method", "path", "code", "service"})
+
+	err := prometheus.Register(counter)
+	if err != nil {
+		registered, ok := err.(prometheus.AlreadyRegisteredError)
+		if !ok {
+			panic(err)
+		}
+		// return existing counter if metrics already registered
+		return registered.ExistingCollector.(*prometheus.CounterVec)
+	}
+
+	return counter
+}