Support date time columns in metrics aggregation

Metrics aggregation code in MutableSegmentImpl (realtime consuming segment code) currently ignores date time columns. If these columns are enabled, then we need to have additional 'key' columns to compare on, before we aggregate metrics.

Start in MutableSegmentImpl, focusing on the realtime consuming segment's metrics aggregation path and how enabled date time columns are currently handled. Trace the existing comparison-key logic, then verify that date time columns contribute additional key columns before aggregation and that metrics aggregate correctly when such columns are enabled.

Written by the indexing model from the issue text.