Summary:
PerformanceStatsEvent declares the metrics we track for performance.
DynamicPerformanceStatsEvent is a new type which extends it by allowing arbitraty key/value pairs to be reported and visualised.
Reviewed By: antonk52
Differential Revision: D47023248
fbshipit-source-id: fadfad79561fca9ae48d0668da3cc62f0d0391d8