prometheus: compose latency metric

Add metric to measure the latency
of requests made to the composer
cloud api.

internal/prometheus/prometheus.go

@@ -32,3 +32,11 @@ var (
 		Help: "total number of failed compose requests",
 	})
 )
+
+var (
+	httpDuration = promauto.NewHistogramVec(prometheus.HistogramOpts{
+		Name:    "composer_http_duration_seconds",
+		Help:    "Duration of HTTP requests.",
+		Buckets: []float64{.025, .05, .075, .1, .2, .5, .75, 1, 1.5, 2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 20},
+	}, []string{"path"})
+)