TUN-8731: Implement diag/system endpoint

## Summary
This PR will add a new endpoint, "diag/system" to the metrics server that collects system information from different operating systems.

Closes TUN-8731

metrics/metrics.go

@@ -15,6 +15,8 @@ import (
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"github.com/rs/zerolog"
 	"golang.org/x/net/trace"
+
+	"github.com/cloudflare/cloudflared/diagnostic"
 )
 
 const (
@@ -52,6 +54,7 @@ func GetMetricsKnownAddresses(runtimeType string) [5]string {
 
 type Config struct {
 	ReadyServer         *ReadyServer
+	DiagnosticHandler   *diagnostic.Handler
 	QuickTunnelHostname string
 	Orchestrator        orchestrator
 
@@ -91,6 +94,8 @@ func newMetricsHandler(
 		})
 	}
 
+	router.HandleFunc("/diag/system", config.DiagnosticHandler.SystemHandler)
+
 	return router
 }