Keep track of how many times a particular query shape was used and its cumulative latency

For index building purposes it would be extremely useful to have a counter per query shape and the associated cumulative latency, even if it's not super accurate.

While it's possible to parse profiler and/or mongod logs to obtain the same information, this generates extra disk IO and requires users to essentially duplicate the effort (since we have to determine the shape and measure the latency in the server anyway).