Add metrics for bucketassign.minibatch.cache.hit.count / cache.miss.count (cache hit ratio)

### Feature Description

**What the feature achieves:**

**Why this feature is needed:**

### User Experience

**How users will use this feature:**
- Configuration changes needed
- API changes
- Usage examples

### Hudi RFC Requirements

**RFC PR link:** (if applicable)

**Why RFC is/isn't needed:**
- Does this change public interfaces/APIs? (Yes/No)
- Does this change storage format? (Yes/No)
- Justification:

Search the Java codebase for bucketassign.minibatch.cache.hit.count and bucketassign.minibatch.cache.miss.count to find the cache instrumentation entry point. Read the surrounding metrics code and existing tests first; done means both cache counts are exposed and their values support the requested cache hit ratio.

Written by the indexing model from the issue text.