This patch reduces the number of metrics that is reported per table, when
the per-table flag is on.
When possible, it moves from time_estimated_histogram and
timed_rate_moving_average_and_histogram to use the unified timer.
Instead of a histogram per shard, it will now report a summary per shard
and a histogram per node.
Counters, histograms, and summaries will not be reported if they were
never used.
The API was updated accordingly so it would not break.
Signed-off-by: Amnon Heiman <amnon@scylladb.com>
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