In Bug 1877258 I added telemetry to two fuses: One for the EmulatesUndefined/IsHTMLDDA fuse, and the other for the OptimizeGetIterator fuse.

After only getting data for the former I explored a hypothesis in Bug 1933830 that we were sensitive to the capitalization being used in UseCounters.conf

This hypothesis seems to have come through -- now that I changed the capitalization I am now able to find data in Looker

Now: I'm not sure where in the system the issue was: I will note that now that I can get information out of looker I appear to -also- be getting data from the past with the incorrect capitalization; so it's possible that there's actually an ingestion bug, but a proximate solution here would be to error when running mach gen-use-counter-metrics if the capitalization is unfavourable.