Use prometheus.Gatherers to avoid duplicating std collectors

Seen on github.com/minio/minio/cmd/metrics.go

This way, the ProcessCollector and the GoCollector will operate on the
Prometheus DefaultRegistry, whose durability is the same as the process.

On the other hand, the registry we instantiate per server will live and die with
it.

I was inclined to implement this because I think that otherwise we ended up with
a redundant set of *Collectors, ones instantiated by us, and the othe ones
instantiated by the prometheus library, as goodies of the DefaultRegistry.

Besides, now I know how to make use of multiple registries (via Gatherers, which
kinda melds them).

api/metricshttp/metricshttp.go

@@ -9,12 +9,16 @@ import (
 
 func NewMetricsHTTP(r *prometheus.Registry) *http.ServeMux {
 	mux := http.NewServeMux()
+	g := prometheus.Gatherers{
+		prometheus.DefaultGatherer,
+		r,
+	}
 	mux.Handle(
 		"/metrics",
 		promhttp.InstrumentMetricHandler(
 			r,
 			promhttp.HandlerFor(
-				r,
+				g,
 				promhttp.HandlerOpts{},
 			),
 		),

server/server.go

@@ -175,15 +175,6 @@ func NewServer(conf *Config) (*Server, error) {
 		server.prometheusRegistry = r
 		metricsMux := metricshttp.NewMetricsHTTP(r)
 
-		r.MustRegister(
-			prometheus.NewProcessCollector(
-				prometheus.ProcessCollectorOpts{},
-			),
-		)
-		r.MustRegister(
-			prometheus.NewGoCollector(),
-		)
-
 		server.metricsServer = newHTTPServer("localhost:9990", metricsMux)
 	}