add min/max/mean/std dev for subscription callbacks

[flynneva]

recommended by @christophebedard

also possibly for publications and timer callbacks, could be related to #37

[christophebedard]

@flynneva I think this can be closed, right?

[flynneva]

@christophebedard long term I think we should add it in to the callback report - it isn't in there yet.

was going to look at ros2_tracing again after our presentation this week

[christophebedard]

oh, alright! I thought it was already in there.