Please describe the feature request.
The micrometer-observation-test dependency does not appear to offer a simple way of asserting on observation events in tests

Rationale
The TestObservationRegistryAssert class handles assertions on what observations have been made, the key values associated with an observation, etc. But it does not appear to provide support for asserting that particular calls to Observation#event(...) have been made.

This is necessary to provide test coverage for observations which raise events.

Additional context
This is based on using version 1.13.2 of io.micrometer:micrometer-observation-test