AFAICT, this is caused by part 6 of the patchset from bug 722034. Basically, we are now only counting per-device cache hit/miss telemetry for the case where we don't have to do any revalidation. Thus, HTTP_DISK_CACHE_DISPOSITION and similar per-device telemetry only includes cache hits.
Created attachment 632532 [details] [diff] [review] Fix cache hit rate telemetry
Nick, I didn't write an automated test for this, but you can verify that it is working by browsing around a little bit and then observing that you have positive counts for values 1, 2, 3, and 4 for HTTP_CACHE_DISPOSITION_2 and the other DISPOSITION_2 probes. I left the max at 5 even though there are only 4 distinct valid values now. I am not sure how much of a difference this makes, but AFAICT it shouldn't hurt.
Comment on attachment 632532 [details] [diff] [review] Fix cache hit rate telemetry Wrong bug.
Created attachment 637778 [details] [diff] [review] Fix cache hit rate telemetry regression You can verify that this patch fixes the problem by observing that, in a build before the patch, HTTP_CACHE_DISPOSITION_2 only has counts for the value 1 after surfing. After surfing for a while with the patch, you will see counts for values 1, 2, 3, 4.