This commit adds a new option optionally disable authentication for the
`/metrics` endpoint in the pulsar-proxy.

Currently, authentication is required for the metrics endpoint when
authentication is enabled, which makes monitoring more difficult.
However, rather than just disable it completely and allow for metrics to
be exposed to any unknown user, this makes it opt in.

It could be argued that it should default to false, but as it is likely
that the proxy is the only component potentially exposed to the public internet, we
default to not exposing data.

Fixes #4920