[feature] Add Metrics to understand Load Balancing Decisions

### Search before asking

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

### Motivation

When pushing pulsar clusters to see maximum performance it is important to understand which metric is causing bundle splits and unloading. The metrics shared only show that the split and/or unload occurred.
1. [bundle unload metrics](../blob/master/site2/docs/reference-metrics.md#bundleunloading-metrics)
2. [bundle split metrics](../blob/master/site2/docs/reference-metrics.md#bundlesplit-metrics)

The logs give values, but it is hard to reason about these or monitor them.

### Solution

Add reason counts for load balancing: bundle splits, and unloads.

1. Bandwidth
2. CPU
3. Direct Memory
4. Memory
5. Admin

### Alternatives

Make the log files be much clearer about which metric has triggered a bundle split or unload.

### Anything else?

This is related to #16691

### Are you willing to submit a PR?

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

Start by reading the bundle unloading and bundle split metrics sections in site2/docs/reference-metrics.md, then review related issue #16691 for context. Identify the load-balancing paths that decide between bandwidth, CPU, direct memory, memory, and admin triggers. Done means reason-specific counts are exposed for bundle splits and unloads and documented with the existing metrics.

Written by the indexing model from the issue text.