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.

Also updated SimpleSpanProcessor and BatchSpanProcessor to explicitly implement and pass the _on_ending method

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

[pentagonal-proboscis]

Has this PR been abandoned or does more work need to be done on it?

I'm looking for exactly this functionality so hope it comes soon.

[majanjua-amzn]

Not abandoned, just struggling to get eyes on it for review. Asked on the #otel-python slack but no luck.

[majanjua-amzn]

Hi @xrmx, could I get some help getting this reviewed? Thanks

[xrmx]

@majanjua-amzn Please fix tox -e lint-opentelemetry-sdk, I think all of them calls for a # pylint: disable=

[majanjua-amzn]

@majanjua-amzn Please fix tox -e lint-opentelemetry-sdk, I think all of them calls for a # pylint: disable=

Wasn't able to run the command locally but this should've fixed it @xrmx

[tammy-baylis-swi]

Nit: this could go to bottom of Unreleased list for chronological order

[majanjua-amzn]

@xrmx workflows passing and got some approvals, could you get to merge the PR? Thanks in advance!