Kotlin-specific Observation API

[ilya40umov]

This PR introduces a couple of extension functions on Observation to help using it from Kotlin code.
It's an alternative version of the following PR: #4772, which would introduce the same API, but on ObservationRegistry-level.

• Observation.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)
• Observation.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"

gh-4754

[ilya40umov]

Hm. Looks like some unrelated tests are failing MongoMetricsCommandListenerTest > shouldCreateFailedCommandMetricWithCustomSettings() . 🤔