Proxy envoy metrics through control plane prometheus endpoint (#709)

* Proxy metrics requests to envoy control plane

internal/telemetry/metrics/providers.go

@@ -2,13 +2,21 @@ package metrics
 
 import (
 	"fmt"
+	"io/ioutil"
 	"net/http"
+	"net/url"
 
 	ocprom "contrib.go.opencensus.io/exporter/prometheus"
 	prom "github.com/prometheus/client_golang/prometheus"
 	"go.opencensus.io/stats/view"
+
+	"github.com/pomerium/pomerium/internal/envoy"
+	log "github.com/pomerium/pomerium/internal/log"
+	"github.com/pomerium/pomerium/internal/urlutil"
 )
 
+var envoyURL = envoy.EnvoyAdminURL
+
 // PrometheusHandler creates an exporter that exports stats to Prometheus
 // and returns a handler suitable for exporting metrics.
 func PrometheusHandler() (http.Handler, error) {
@@ -26,7 +34,13 @@ func PrometheusHandler() (http.Handler, error) {
 	}
 	view.RegisterExporter(exporter)
 	mux := http.NewServeMux()
-	mux.Handle("/metrics", exporter)
+
+	envoyMetricsURL, err := urlutil.ParseAndValidateURL(fmt.Sprintf("%s/stats/prometheus", envoyURL))
+	if err != nil {
+		return nil, fmt.Errorf("telemetry/metrics: invalid proxy URL: %w", err)
+	}
+
+	mux.Handle("/metrics", newProxyMetricsHandler(exporter, *envoyMetricsURL))
 	return mux, nil
 }
 
@@ -37,3 +51,33 @@ func registerDefaultViews() error {
 	}
 	return view.Register(views...)
 }
+
+// newProxyMetricsHandler creates a subrequest to the envoy control plane for metrics and
+// combines them with our own
+func newProxyMetricsHandler(promHandler http.Handler, envoyURL url.URL) http.HandlerFunc {
+
+	return func(w http.ResponseWriter, r *http.Request) {
+		defer promHandler.ServeHTTP(w, r)
+
+		r, err := http.NewRequestWithContext(r.Context(), "GET", envoyURL.String(), nil)
+		if err != nil {
+			log.Error().Err(err).Msg("telemetry/metrics: failed to create request for envoy")
+			return
+		}
+
+		resp, err := http.DefaultClient.Do(r)
+		if err != nil {
+			log.Error().Err(err).Msg("telemetry/metrics: fail to fetch proxy metrics")
+			return
+		}
+		defer resp.Body.Close()
+
+		envoyBody, err := ioutil.ReadAll(resp.Body)
+		if err != nil {
+			log.Error().Err(err).Msg("telemetry/metric: failed to read proxy metrics")
+			return
+		}
+
+		w.Write(envoyBody)
+	}
+}