RUMM-2226: Fix handling of span scope in RxJava chain

[0xnm]

What does this PR do?

This change fixes the usage of span scope in the RxJava chain.

Documentation of ScopeManager#activate says that "It is a programming error to neglect to call {@link Scope#close()} on the returned instance.". So to comply with that code now stores Scope returned by ScopeManager#activate call and calls Scope#close when related span is finished.

Without doing that the scope is always alive and further scopes created will get it as a parent (and new spans will have finished span as a parent).

Also examples in the docs are updated: code example for Java is slightly different from Kotlin, because Java allows having only final variables being captured, so ThreadLocal is introduced. Normally ThreadLocal could be used in Kotlin as well, because particular Scope is bound to the thread, so having a call on the instance from another thread won't have any effect, but since Kotlin allows any variable to be capture by closure, the code sample is simplified there.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[alai97]

Looks great, thanks!