See Bug 1584892 Comment 13. Workers do not currently provide telemetry from their JS engine.

I'm going to set the MemShrink "thing" in the whiteboard field because AIUI this could lead to us not being aware of changes to memory usage due to workers, I'd like that team to see it.

Is this something that should live in CycleCollectedJSRuntime? (Right now the JS telemetry happens in XPCJSRuntime[1].)

1: https://searchfox.org/mozilla-central/rev/b6f088f2f68d2a8ae0b49d6c8fbd7cbd3a65fa5b/js/xpconnect/src/XPCJSRuntime.cpp#3081 hooks up
https://searchfox.org/mozilla-central/rev/b6f088f2f68d2a8ae0b49d6c8fbd7cbd3a65fa5b/js/xpconnect/src/XPCJSRuntime.cpp#2658

That would probably be reasonable.

For the cycle collector, we have an awkward macro to let us collect main thread and worker telemetry:
https://searchfox.org/mozilla-central/rev/b6f088f2f68d2a8ae0b49d6c8fbd7cbd3a65fa5b/xpcom/base/nsCycleCollector.cpp#510

Something like that might be worth considering.