shutdown manager: use filter on downstream_cx_active stats url (#6523)

Add a filter to prometheus stats url so that
only the desired stats are returned. This improves
the performance especially in clusters with high
number of routes

Signed-off-by: Ahmad Karimi <ak12hastam@gmail.com>

changelogs/unreleased/6523-therealak12-small.md

@@ -0,0 +1 @@
+Improve shutdown manager query to the Envoy stats endpoint for active connections by utilizing a regex filter query param.

cmd/contour/shutdownmanager.go

@@ -31,7 +31,9 @@ import (
 )
 
 const (
-	prometheusURL      = "http://unix/stats/prometheus"
+	// The prometheusURL is used to fetch the envoy metrics. Note that the filter
+	// value matches Envoy's raw stat names (i.e. those on the `/stats/` endpoint).
+	prometheusURL      = "http://unix/stats/prometheus?filter=^http\\..*\\.downstream_cx_active$"
 	healthcheckFailURL = "http://unix/healthcheck/fail"
 	prometheusStat     = "envoy_http_downstream_cx_active"
 )