Reuse http.DefaultTransport in UIMetricsProxy (#14521)

http.Transport keeps a pool of connections and should be reused when possible. We instantiate a new http.DefaultTransport for every metrics request, making large numbers of concurrent requests inefficiently spin up new connections instead of reusing open ones.

.changelog/14521.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+ui: Reuse connections for requests to /v1/internal/ui/metrics-proxy/
+```

agent/agent.go

@@ -941,6 +941,7 @@ func (a *Agent) listenHTTP() ([]apiServer, error) {
 			srv := &HTTPHandlers{
 				agent:          a,
 				denylist:       NewDenylist(a.config.HTTPBlockEndpoints),
+				proxyTransport: http.DefaultTransport,
 			}
 			a.configReloaders = append(a.configReloaders, srv.ReloadConfig)
 			a.httpHandlers = srv

agent/http.go

@@ -81,6 +81,10 @@ type HTTPHandlers struct {
 	configReloaders []ConfigReloader
 	h               http.Handler
 	metricsProxyCfg atomic.Value
+
+	// proxyTransport is used by UIMetricsProxy to keep
+	// a managed pool of connections.
+	proxyTransport http.RoundTripper
 }
 
 // endpoint is a Consul-specific HTTP handler that takes the usual arguments in

agent/ui_endpoint.go

@@ -771,6 +771,7 @@ func (s *HTTPHandlers) UIMetricsProxy(resp http.ResponseWriter, req *http.Reques
 		Director: func(r *http.Request) {
 			r.URL = u
 		},
+		Transport: s.proxyTransport,
 		ErrorLog: log.StandardLogger(&hclog.StandardLoggerOptions{
 			InferLevels: true,
 		}),