Let us move first paint and first contentful paint to the navtiming schema to make things simpler.
- Change how we collect the metric in the navigation timing extension https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/extensions/NavigationTiming/+/refs/heads/master - we should collect the metrics at the same time as we get the Navigation Timing data.
- Then we need to make sure that the data is stored in the navigation timing schema https://gerrit.wikimedia.org/r/plugins/gitiles/schemas/event/secondary/+/refs/heads/master/jsonschema/analytics/legacy/navigationtiming/ so we need to ad the fields missing there. Also remove reference to the paint timing schema and deprecate it.
- Add so we take care of the data when it arrives and send it to Graphite/Prometheus. That happens in navtiming.py: https://gerrit.wikimedia.org/r/plugins/gitiles/performance/navtiming/ - do it in two steps, one we add the functionality to push the data following the schema and follow the same structure as before in Prometheus/Graphite. And then next step when everything is live, we can remove the old push of the data using the Paint Timing API.