Right now we're rejecting a unknown portion of browsers from the metric gathering based on our `isCompliant` check. Per comments on T104902, this is potentially a significant cut.
The goal here is to get insight into how big of a chunk we're talking about.
When a page view falls in the Navigation Timing sampling, we should always emit an event. Either with Schema:NavigationTiming if all the data is complete and usable. And otherwise, an error increment of sorts.
There are two parts to this:
* Track errors from EventLogging itself (such as for url size, T112592).
* Track rejection from our own code for non-compliant browsers.
The total sum of different error types should be tracked on the dashboard at <https://grafana.wikimedia.org/#/dashboard/db/performance-metrics>, along side the existing tracking for schema sample rate (which tracks rate of valid/successful events).