Certain table metrics not released when table is dropped.

The following metrics are not released when a table is dropped:

• CasCommitLatency
• CasCommitTotalLatency
• CasPrepareLatency
• CasPrepareTotalLatency
• CasProposeLatency
• CasProposeTotalLatency
• ReadRepairRequests
• ShortReadProtectionRequests

This can be verified by inspecting the list of exported MBeans after creating a table then dropping it.

The result is that if a table with the same name is re-created then the above metrics wont be updated - internally a javax.management.InstanceAlreadyExistsException is thrown by the MBeanServer when the new table's metric is registered, but this exception is silently ignored by org.apache.cassandra.metrics.CassandraMetricsRegistry#registerMBean

The above metrics were added in 2014/2015, so this issue probably affects all versions released since then.

Attached a patch that releases/removes the metrics when the table is dropped.