Summary:
I was trying to plot some graphs based on our background plugin stats, grouped per plugin, but discovered that we can't exploded or group on json object keys, so separated the data into separate `plugin-stats-plugin` events, as is done with the `time-spent` event.
Also fixed:
* grouping stats per plugin, rather than per plugin method, which was to fine-grained
* added `bytesReceived` field to the cumulative event `plugin-stats`
Reviewed By: jknoxville
Differential Revision: D21046016
fbshipit-source-id: 1043612064921cf6427d5b3bbee10b76776df39e
8b75e81da5