This patch adds a few data structure for derived and accumulative
statistics that are similiar to the yammer implementation used by the
JMX.
It also adds a plus operator to histogram which cleans the histogram
usage.
moving_average - An exponentially-weighted moving average. calculate an event rate
on a given interval.
rate_moving_average and timed_rate_moving_average - Calculate 1m, 5m and
15m ewma an all time avrage and a counter.
rate_moving_average_and_histogram and
timed_rate_moving_average_and_histogram - Combines a histogram with a
rate_moving_average. It also expose a histogram API so it will be an
easy task to replace a histogram with a
timed_rate_moving_average_and_histogram.
Signed-off-by: Amnon Heiman <amnon@scylladb.com>
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