feat(lib): Add anubis_proxied_requests_total metric (#570)

Adds a new counter metric which counts the number of requests proxied to
the upstream target, labeled with the host name of the request.

lib/anubis.go

@@ -56,6 +56,11 @@ var (
 		Help:    "The time taken for a browser to generate a response (milliseconds)",
 		Buckets: prometheus.ExponentialBucketsRange(1, math.Pow(2, 18), 19),
 	})
+
+	requestsProxied = promauto.NewCounterVec(prometheus.CounterOpts{
+		Name: "anubis_proxied_requests_total",
+		Help: "Number of requests proxied through Anubis to upstream targets",
+	}, []string{"host"})
 )
 
 type Server struct {

lib/http.go

@@ -147,6 +147,7 @@ func (s *Server) ServeHTTPNext(w http.ResponseWriter, r *http.Request) {
 			web.Base("You are not a bot!", web.StaticHappy()),
 		).ServeHTTP(w, r)
 	} else {
+		requestsProxied.WithLabelValues(r.Host).Inc()
 		s.next.ServeHTTP(w, r)
 	}
 }