This adds the latency histogram to the column_family swagger
definitions.
The definitions are based on the ColumnFamilyMetrics.
It adds the following commands:
get_read_latency_histogram
get_all_read_latency_histogram
get_write_latency_histogram
get_all_write_latency_histogram
Signed-off-by: Amnon Heiman <amnon@cloudius-systems.com>