Polishing Observation API

[jonatan-ivanov]

• DocumentedObservation.createNotStarted -> observation (to make it consistent with others)
• ObservationRegistry.observationConvention: removed varargs
• Make getObservationConvention and isObservationEnabled package-private in the ObservationRegistry (should not be used)
• Make getError return Throwable

[jonatan-ivanov]

Let's discuss if we want to rename this class.
@shakuzen Did you get some feedback on this?

[ttddyy]

I thought we rename existing observation to createNotStarted?
Either way, it is better to be unified.

[jonatan-ivanov]

Yepp, that's why the let's discuss comment.
I think:

1. If we want to keep the class name as-is: it is better to rename everything to createNotStarted
2. If we want to rename the class to ObservationDocumentation (or similar): it is better to rename everything to observation
3. If we want to keep changes minimal: what I did

I think the best would be # 2 but it is also the most invasive.

[shakuzen]

I think ObservationDocumentation better expresses what the interface is for and makes it clear that it is not an Observation itself. The feedback I got so far was that they agreed with that. I think it will be easier to consider that change in a separate pull request, though, so we don't block making the rest of these changes.

[marcingrzejszczak]

BTW if we're chaning the DocumetedObservation to ObservationDocumentation we also need to change the DocumentedMeter to MeterDocumentation and in tracing the DocumentedSpan to SpanDocumentation. Also the code in micrometer-docs-generator needs to be updated

[sonatype-lift]

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