Micrometer Observation Support

I'm a co-maintainer of [Spring Cloud Sleuth](https://github.com/spring-cloud/spring-cloud-sleuth/) and [Micrometer](https://micrometer.io/) projects (together with @shakuzen and @jonatan-ivanov).

[Micrometer Observation](https://micrometer.io/docs/observation) is part of the Micrometer 1.10 release and [Micrometer Tracing](https://micrometer.io/docs/tracing) is a new project. The idea of Micrometer Observation is that you instrument code once but you get multiple benefits out of it - e.g. you can get tracing, metrics, logging or whatever you see fit).

Since this project supports OpenCensus that is deprecated I was curious if there's interest in adding Micrometer Observation support so that except for metrics, spans could be created and tracing context propagation could happen too. Via Micrometer Tracing one can use OpenTelemetry or OpenZipkin Brave Tracer, but with the handler mechanism the possibilities are endless :)

If there's such interest we could provide a PR to add support for that.