Due to elastic#608 it's likely we need to benchmark scenarios without using
the node-stats telemetry device. At the same time we want to get a
general idea of how many refreshes/merges/flushes happened (in total)
by accessing the index stats.

Add total count for merges/refresh/flush in summary output; this is
collected from `_all/primaries` in `_stats`.

Also modify summary description to clarify the values are totals from
primary shards.

Finally fix bug where index stats where time/count == 0 got skipped
from the summary.

Closes elastic#614

And group total counts directly below total times in all summary reports.

Use cumulative instead of total for summary reports and comparisons
and clarify that min/median/max are across primary shards.

Relates: elastic/elasticsearch#35594 (comment)