This is required to have a performant "Feedback" RPC.

General proposal:

message FeedbackRequest {
  string project
  string category
  string start_time
  string end_time
}

message FeedbackResponse {
  int32 comments
  int32 not_useful_reports
}

General proposal:
 - The cron job could aggregate at per-day resoloution to start with, but when different levels of precision are needed they could be added in a hierarchical manner, e.g. with hour resolution entities being children of day resolution entities.
 - The start_time and end_times from the request could potentially be rounded, e.g. 2006-01-02T08:04:05Z could be treated the same as 2006-01-02T00:00:00Z.
 - The cron job could be called something like aggregate_feedback, and the tracked Datstore model could be called FeedbackStats or something else, and the Feedback RPC handler would just fetch those entities with a query.

Steps that this could be broken into:
 - Add project field to FeedbackRequest.
 - Modify datastore schema.
 - Add cron job and turn it on.
 - Modify Feedback request.