Add WebSocket Proxy Prometheus Metrics Endpoint

### Search before asking

- [X] I searched in the [issues](https://github.com/apache/pulsar/issues) and found nothing similar.

### Motivation

Metrics are essential to all production deployments.

### Solution

Expose a metrics endpoint. This will require formatting the metrics, and then exposing them. Typically, these metrics are exposed at the `/metrics` endpoint.

### Alternatives

There is currently and endpoint for `/admin/v2/proxy-stats/metrics`, but that exposes metrics in a JSON format. It looks like the logic for those metrics could easily be converted into correct Prometheus metrics.

### Anything else?

_No response_

### Are you willing to submit a PR?

- [x] I'm willing to submit a PR!

Start by locating the proxy implementation for /admin/v2/proxy-stats/metrics and the code that formats its JSON response. Compare the existing metrics with Prometheus exposition requirements and determine where a /metrics endpoint should be exposed. Done means the proxy serves the converted metrics at /metrics and coverage verifies the endpoint and format.

Written by the indexing model from the issue text.