Get project factory merge rate

Counts this project's factory PRs that GitHub webhooks record as closed, by the PR’s close time, in the requested window (filter[timestamp][gte] to the exclusive filter[timestamp][lt], at most 90 days, the last 7 days when neither is given) and in the window of the same length just before it. A factory PR is one a project run produced. closedPrs counts them merged or not, and mergedPrs those that merged. trackedSince is when the repository’s record of PRs starts, null before its first. series has one point per UTC day that the current window touches, zeros included, and its points add up to current.