Add Agent diagnostic hook to dump beat metrics (elastic#38115)

Register an Agent diagnostic hook that will dump all metrics registered into
the default monitoring namespace as well as expvar. The hook is registered
within libbeat so this change will affect all Beats operating under Agent.

The file will be named beat_metrics.json and will contain a single pretty JSON
object.

Closes elastic#37929

libbeat/cmd/instance/beat.go

@@ -893,6 +893,16 @@ func (b *Beat) configure(settings Settings) error {
 
 	b.Manager.RegisterDiagnosticHook("global processors", "a list of currently configured global beat processors",
 		"global_processors.txt", "text/plain", b.agentDiagnosticHook)
+	b.Manager.RegisterDiagnosticHook("beat_metrics", "Metrics from the default monitoring namespace and expvar.",
+		"beat_metrics.json", "application/json", func() []byte {
+			m := monitoring.CollectStructSnapshot(monitoring.Default, monitoring.Full, true)
+			data, err := json.MarshalIndent(m, "", "  ")
+			if err != nil {
+				logp.L().Warnw("Failed to collect beat metric snapshot for Agent diagnostics.", "error", err)
+				return []byte(err.Error())
+			}
+			return data
+		})
 
 	return err
 }