Track the number of commit retry attempts in operationMetrics for write operations.

## Changes

1. Add num_retries field to WriteMetrics struct
   - Initialized to 0 when metrics are created
   - Uses #[serde(default)] for backward compatibility with old delta logs

2. Add update_retry_count_in_bytes() helper method
   - Efficiently updates only the commitInfo action's operationMetrics
   - Avoids re-serializing all Add/Remove actions (important for high-concurrency scenarios)
   - Updates num_retries field right before each commit attempt

3. Update retry loop in PreparedCommit
   - Calls helper method before each write_commit_entry attempt
   - Ensures accurate retry count is persisted to delta log

## Performance

- Minimal overhead: only parses/serializes commitInfo (~1-2KB), not all actions
- Critical for high-concurrency workloads where retries are common

## Result

Delta log now includes num_retries in operationMetrics:
{
  "operationMetrics": {
    "num_added_files": 5,
    "num_retries": 2,
    ...
  }
}