Adds:
- `build_stats_timeseries()` for build counts over time
- `duration_percentiles_timeseries()` for P50/P95/P99
- `system_distribution()` for per-system counts
and of course, REST endpoints for `/api/v1/metrics/timeseries/*`. This
is a good start for data visualisation. My professors would be proud.
Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I3c0b9d14592945a661af77b7edf338a86a6a6964
