This patch adds the helper function that are used to sum the
rate_moving_average and rate_moving_average_and_histogram.
The current sum functionality for histogram was modified to support
rate and histogram but return a histogram. This way current endpoints
would continue to behave the same.
It also cleans the histogram related method by using the plus operator
in the histogram.
Signed-off-by: Amnon Heiman <amnon@scylladb.com>
f6a5a4e3da