Kotlin-specific ObservationRegistry API

[ilya40umov]

This PR introduces a couple of extension functions on ObservationRegistry to help using Observation API from Kotlin code.

• ObservationRegistry.observeAndGet allows to observe a non-suspending block of code:
  • this method is analogous to Observation.observe, but accepts a block of code instead of expecting a Java Supplier
  • this method will work with both nullable and non-nullable generic type (while usage of Supplier makes Kotlin compiler think the result of the nested block is always nullable)
• ObservationRegistry.observeAndAwait allows to observe a suspending block of code:
  • this method is again similar to Observation.observe, but will accept a suspending block of code
  • by using openScope().use { observationRegistry.asContextElement() } it manages to create an instance of KotlinObservationContextElement with the new observation captured as "current"

Initial idea: Why add those to ObservationRegistry and not to Observation? Mostly for consistency. I.e. observeAndAwait needs access to ObservationRegistry anyway, so it's easy to add it to the registry as an extension. While observeAndGet could indeed be made into an extension function on Observation, however it still needs to be called something other than observe, as otherwise Intellij can't resolve which observe version the code is referring to.

Update: I realized that Observation also contains registry it was created for, so I created an alternative PR which adds both of the extension functions to Observation class: #4823

gh-4754

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[ilya40umov]

@shakuzen would you be the one to ask for review on this PR? 🙏

[shakuzen]

would you be the one to ask for review on this PR?

I will try to find someone to take a look. Some folks are off this week, though.

[ilya40umov]

I realized that Observation also contains the registry it was created for, so I created an alternative PR which adds both of the extension functions to Observation class as a possible alternative: #4823

Feel free to choose which one would make more sense for Micrometer project to include.