As a recommendation from performance review (T246707), we should track the rate at which relevant Echo events/notifications are occurring.
This was originally instrumented with event logging (see Schema:Echo), but that was disabled because it was seen as redundant with data already in Echo's DB tables: https://gerrit.wikimedia.org/r/c/operations/mediawiki-config/+/274345
Q: Do we have a good way of using the rate of inserts on a given DB table as a metric? A: It appears not.