Log when an event was triggered but no observer consumed it

## What problem does this solve or what need does it fill?

We can easily see where an event was triggered from, but not that it didn't cause any observers to run. We should add a (feature flagged) debug message for when an event is triggered, but no observer consumes it.

## What solution would you like?

Log when an event is not observed by anything. This should be behind a non-default feature flag. We should also consider adding an umbrella "debug" feature flags for all our various ones.