feat: implement on ending in span processor

[majanjua-amzn]

Description

SpanProcessor.onEnding() Specification

This PR serves to implement the in-development feature onEnding() for the span processor. This will allow users to make modifications to the span before it becomes immutable when onEnd() is reached.

Loosely following implementation in Java repo: open-telemetry/opentelemetry-java#6367

Fixes: N/A

Type of change

New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Callout: Maintained previous unit test that validated span processor behaviour without onEnding
• Added a unit test to ensure onEnding is called in the correct order (in between onStart and onEnd)
• Added new unit tests to ensure onEnding is called appropriately (once per Span.End())

Does This PR Require a Contrib Repo Change?

No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated