[Core] Support Truncate(0) for metrics

[KartikKapur]

Background
At Pinterest, we've started utilizing iceberg metrics considerably for offline validation as well as query speedups. Counts are consistently useful for all columns and upper/lowerbound is useful for numeric columns.; however, for struct columns (typically objects with encoded strings), ranges are relatively useless and just cause space overhead with potential driver OOM. There isn't an easy way to specify metrics per data type so wanted to contribute a solution which allows to specify truncate limit as 0 which just causes strings and binaries to be fully truncated.

Changes
Updated checks in Truncate MetricsMode to allow for non-negative rather than strictly positive length. Updated appropriate checks in order to only input into bounds map if non null

Tests

Added appropriate unit tests