CASSANDRA-21320: add rows mutated per write request histogram

[shiroonigami23-ui]

What is this

Implements a table-level histogram for write request size in rows.

JIRA

• CASSANDRA-21320

Changes

• Added RowsMutatedPerWriteHistogram to TableMetrics.
• Added matching keyspace histogram RowsMutatedPerWriteHistogram in KeyspaceMetrics.
• Updated write request metric recording to update this histogram once per write request per table.
  • For multi-mutation requests (e.g., batches), rows are aggregated per table and recorded once.
  • For single PartitionUpdate, one histogram update is recorded for that request.
• Extended ClientRequestRowAndColumnMetricsTest with assertions for the new table histogram.

Testing

• Unable to run unit tests in this environment because ant is not installed (ant: command not found on Windows shell).
• Verified changed call paths and metric wiring via code inspection and diff checks.