Handle struct-level metrics from non-Parquet writers

### Feature Request / Improvement

https://github.com/apache/iceberg/pull/17412 removed struct-level metrics from `TestInclusiveMetricsEvaluator` to align with the current behavior of Parquet. However, the spec doesn't technically prevent a writer from emitting struct-level metrics for v1-v3. Also the v4 content stats don't keep track of struct-level metrics.
The goal here is to investigate how we want to deal with struct-level metrics in general and especially if non-Parquet writers emit them

### Query engine

None

### Willingness to contribute

- [ ] 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 reading TestInclusiveMetricsEvaluator and the changes in pull request 17412, then compare the specification's v1-v3 rules with v4 content stats. Investigate how struct-level metrics from non-Parquet writers should be handled; done means the project has an agreed behavior for these metrics, backed by appropriate evaluator coverage.

Written by the indexing model from the issue text.