Observation auto-configuration tries to configure metrics and tracing when there's no Tracer bean

[jonatan-ivanov]

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This PR is trying to address three potential issues with the ObservationAutoConfiguration:

• Users can't replace/get rid of DefaultMeterObservationHandler
• If tracing is configured DefaultMeterObservationHandler should be wrapped to TracingAwareMeterObservationHandler so that tracing is interoperable with metrics
• Registration order of handlers should be tracing first then metrics

This PR depends on TracingAwareMeterObservationHandler which is only available in a SNAPSHOT release of Micrometer Tracing so either we wait till the RC release of it or we can do an off-cycle milestone release.