Add added-puffin-files write metric

### Feature Request / Improvement

This distinguishes logical delete entries from physical Puffin files for DV-based row-level writes, improving MERGE observability and troubleshooting.

Before this improvement:

After this improvement:

### Query engine

Spark

### Willingness to contribute

- [x] I can contribute this improvement/feature independently
- [ ] I would be willing to contribute this improvement/feature with guidance from the Iceberg community
- [ ] I cannot contribute this improvement/feature at this time

Start by locating the existing write metrics and the Spark row-level write path that handles Puffin files. Compare the current logical delete and physical file counts with the requested added-puffin-files metric, then verify that MERGE observability distinguishes them as shown in the issue.

Written by the indexing model from the issue text.