[4920][proxy] Add option to disable authentication for proxy /metrics

[addisonj]

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

Verifying this change

• Make sure that the change passes the CI checks.

This change should ideally have tests, but there currently aren't any tests for any proxy endpoint other than client proxy.

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): no
• The public API: no
• The schema: no
• The default values of configurations: yes
• The wire protocol: no
• The rest endpoints: no
• The admin cli options: no
• Anything that affects deployment: no

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? docs
• If a feature is not applicable for documentation, explain why?
• If a feature is not documented yet in this PR, please create a followup issue for adding the documentation

[sijie]

run cpp tests