Handle struct-level metrics from non-Parquet writers

[nastra]

Feature Request / Improvement

#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