HTTPCLIENT-2233- Add Metrics Collection for IOReactor Thread Pool

[arturobernalg]

This PR addresses HTTPCLIENT-2233 by implementing a mechanism for collecting metrics on the IOReactor thread pool in Apache HttpClient. The changes introduce an IOReactorMetricsListener interface and a LoggingIOReactorMetricsListener implementation to monitor and report key metrics for better insight into reactor performance and load.

[ok2c]

@arturobernalg

1. The original reporter of HTTPCLIENT-2233 never properly specified what kind of metrics they needed. Can we be sure this is what users need or we end up adding more complexity with little benefit?
2. If you decide to proceed with this PR please add missing @since tags. There is a lot of new methods
3. Please note the release series of core is 5.4 not 5.5. Please correct @since 5.5 tags

[arturobernalg]

@arturobernalg

1. The original reporter of [HTTPCLIENT-2233](https://issues.apache.org/jira/browse/HTTPCLIENT-2233) never properly specified what kind of metrics they needed. Can we be sure this is what users need or we end up adding more complexity with little benefit?

2. If you decide to proceed with this PR please add missing ` @since` tags. There is a lot of new methods

3. Please note the release series of core is 5.4 not 5.5. Please correct ` @since 5.5` tags

@ok2c
I understand the original request was not very specific, so I tried to include metrics that would be valuable for users monitoring I/O reactor performance. The metrics added focus on active thread count, pending connections, thread pool saturation, and queue wait time, which are key indicators for system health and performance in an async environment. I believe these metrics can provide meaningful insights for users needing more control over their thread pool's performance.

[ok2c]

I believe these metrics can provide meaningful insights for users needing more control over their thread pool's performance.

@arturobernalg All right. Please do add missing @since tags, though.

[ok2c]

@arturobernalg Would you agree to move metrics collection inside if (threadPoolListener != null) {? There is no need to collect them if there is no listener.

[arturobernalg]

@ok2c done

[ok2c]

@arturobernalg I added missing @since tags and tweaked the sequence of constructor parameters a tiny bit.