Metric from bookkeeper client to detect ensemble creation failure

### Search before asking

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

### Motivation

We hit one bug related to racks-placement - https://github.com/apache/pulsar/issues/23282. As the enforceMinRack was enabled, the ensemble creation started failing.
There should be some metrics to detect the ensemble creation failure.

### Solution

_No response_

### Alternatives

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

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

Start by reading the motivation and the linked racks-placement issue #23282, focusing on how enforceMinRack can make ensemble creation fail. The issue names no files, tests, or entry points and gives no proposed metric or acceptance criteria, so the metric design and completion conditions need clarification first.

Written by the indexing model from the issue text.