SpringBoot Automated registration of ObservationAwareSpanThreadLocalAccessor via the ContextPropagation API

related issue https://github.com/micrometer-metrics/tracing/issues/206

Comment From: philwebb

We're currently blocked on Reactor/Micrometer agreeing on a solution that works with multiple ObservationRegistry instances. This is particularly important when running tests with the context cache.

Comment From: philwebb

Design wise, this might be a similar problem to #34201