feat: add metrics

[chalabi2]

This PR adds rate limit metrics requested in #79 to Caddy's internal metrics registry

[mohammed90]

This will stop working on config reload, and no metrics will be registered or emitted. Unfortunately, the only way to manage it for now is to check for duplicate registration error, and ignore it.

[chalabi2]

Thanks for the review! I updated it as per your request.

[mohammed90]

Now it will panic if the rate_limiter is used multiple times in Caddy because internally it calls MustRegister, and prometheus will see an existing registration

[chalabi2]

Thanks!

• Removed sync.Once to allow re-registration on config reloads.
• Now directly call initializeMetrics(reg) in registerMetrics.
• globalMetrics is only set if nil, keeping metrics functional across reloads.
• Duplicate registration errors are now ignored (returning nil as suggested).

This way, metrics continue working after reloads without panics, and Prometheus should handle any duplicate registrations gracefully.

[mohammed90]

I'm trying to address the issue of duplicate registration in caddyserver/caddy#7051. Do you have any feedback to give on that there?

[chalabi2]

I'm trying to address the issue of duplicate registration in caddyserver/caddy#7051. Do you have any feedback to give on that there?

Hey sorry for the delay.

I dont have any explicit feedback on your recommended solution in the PR you linked. Sorry this is my first foray into using caddy in production and just noticed some things were missing for my specific use case.