Track event count & API response times

[krancour]

I feel like I've seen API response times degrade as the number of events gets "very large" (hundreds of thousands; really that's not huge).

I'd like to have data to back this up.

Initially, I felt a graph of the total number of events in the system was not especially useful, but if we can compare that graph to API response times, it is useful as we'd be able to either confirm or refute the correlation.