ObservationValidator fails with NullObservation

[chemicL]

Describe the bug

When empty scopes are present thanks to NullObservation, the validation with ObservationValidator fails.

To Reproduce

micrometer-metrics/tracing#810

Expected behavior

NullObservation doesn't cause ObservationValidator to fail.

Additional context

The issue is that NullObservation doesn't launch handlers for most events (e.g. start) but it launches handlers for scope opening and closing events. Combined with ObservationValidator being implemented as a handler this always causes NullObservation to trigger a failure.

Ideas, notes

It is crucial that NullObservation doesn't do unnecessary work as it is often on the hot path in reactive chains.

Therefore, perhaps it's worth considering an alternative implementation only created at test runtime which fires up all handlers. That code would need to interact with ObservationThreadLocalAccessor.

Another idea would be to skip validation on NullObservation, e.g. by providing

context -> !"null".equals(context.getName())

as supportsContextPredicate argument to ObservationValidator.

[shakuzen]

A new NullContext was introduced to make the conditional less brittle for ignoring NullObservation in the ObservationValidator.