Add options for server

[rentziass]

This would give a user the possibility of creating a new server and fully rely on the package sensible defaults, while giving the power the change what they want to change. And that comes with the benefit that adding more fields or options for a Server doesn't necessarily add more complexity or checks to New itself

A few examples of how this would look from a user perspective:

flexmetrics.New()
flexmetrics.New(flexmetrics.WithAddr(os.Getenv("MY_OWN_ENV_VAR"))
flexmetrics.New(flexmetrics.WithServer(myCustomerOPProprietaryServer))