This implements the RPC for collecting table stats.
Since both leaving and pending replica can be accounted during
tablet migration, the RPC handler will look at tablet transition
info and account only either leaving or replica based on the
tablet migration stage. Replicas that are not leaving or
pending, of course, don't contribute to the anomaly in the
reported size.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
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