Prometheus support (#450)

Co-authored-by: nils måsén <nils@piksel.se>
Co-authored-by: MihailITPlace <ya.halo-halo@yandex.ru>
Co-authored-by: Sebastiaan Tammer <sebastiaantammer@gmail.com>

pkg/api/metrics/metrics.go

@@ -0,0 +1,27 @@
+package metrics
+
+import (
+	"github.com/containrrr/watchtower/pkg/metrics"
+	"net/http"
+
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+// Handler is an HTTP handle for serving metric data
+type Handler struct {
+	Path    string
+	Handle  http.HandlerFunc
+	Metrics *metrics.Metrics
+}
+
+// New is a factory function creating a new Metrics instance
+func New() *Handler {
+	m := metrics.Default()
+	handler := promhttp.Handler()
+
+	return &Handler{
+		Path:    "/v1/metrics",
+		Handle:  handler.ServeHTTP,
+		Metrics: m,
+	}
+}