Disable or have a fixed port for the metrics server on the operator

[karampok]

This part on the code
https://github.com/metallb/metallb-operator/blob/main/main.go#L129-L132

		metricsAddr          = flag.String("metrics-addr", ":0", "The address the metric endpoint binds to.")

...
	mgr, err := ctrl.NewManager(ctrl.GetConfigOrDie(), ctrl.Options{
		Scheme: scheme,
		Metrics: metricsserver.Options{
			BindAddress: *metricsAddr,
		},

when ":0" it opens a random high port in the container

 ss -natp |grep LISTEN
LISTEN    0      4096                     *:8080                    *:*
LISTEN    0      4096                     *:33391                   *:*
LISTEN    0      4096                     *:9443                    *:*

which is not ideal. We should improve that part either

1. Disable fully the metrics server when there is no argument
2. Fix a high random static port, so at least a grep search will show what it is
3. Something else