metrics should be optional in observability layer

With the introduction of microprofile, one could provision both cloud-server,microprofile and exclude any microprofile-* layer.
config and metrics being not optional in observability, they can't be excluded from cloud-server although they can with other server (jaxrs, datasources).
config is required by health, so can't be made optional.

So to offer enough flexibility (eg: exclude metrics but provision health) and an aligned experience across the server layers, metrics should be optional in observability.