Instrument Mango execution stats
Adds metrics for mango execution statistics:
* total docs examined (counter). Note that for non-quorum doc
reads this needs to be counted at the shard level and not be the
mango query coordinator
* total results returned (counter)
* query time (histogram)
* mango:selector evaluations (may be queries or _changes feeds)
For mrview-based queries which don't use quorum reads (the default),
total docs examined should equal mango:selector evaluations. However,
there are cases where mango selectors are evaluated outside of query
time e.g. partial indexing, selector-based _changes which make it
useful to maintain a distinct counter.
5 files changed
