[Backport MB-51589 to 7.1.1] Implement node level scan stats

As of 7.1, without looking at stats for individual indexes, it is difficult to tell if overall scan load has changed. With support of 10K indexes, checking stats for individual indexes has become difficult. It is useful to have node level scan stats like num_rows_scanned, num_requests, num_rows_returned etc.

Also, it will be useful to add aggregated stats to output of /api/v1/stats.