[cassandra] Add read and write latency ColumnFamily metrics
[cassandra] Add read and write latency ColumnFamily metrics

Read and Write latency are not only a way to figure out how long it takes to run the specified operation against the given columnfamily, the count metric is also the only way to figure out how often we're reading from and writing to a column family.
This adds the following attributes for read and write latency:
- Count (frequency of reads/writes)
- Mean latency
- 99th percentile latency
- Max latency

Wattpad/dd-agent,manolama/dd-agent,cberry777/dd-agent,cberry777/dd-agent,Wattpad/dd-agent,jyogi/purvar-agent,takus/dd-agent,manolama/dd-agent,jyogi/purvar-agent,cberry777/dd-agent,takus/dd-agent,jyogi/purvar-agent,jyogi/purvar-agent,takus/dd-agent,manolama/dd-agent,manolama/dd-agent,Wattpad/dd-agent,manolama/dd-agent,takus/dd-agent,cberry777/dd-agent,takus/dd-agent,Wattpad/dd-agent,cberry777/dd-agent,jyogi/purvar-agent,Wattpad/dd-agent