Clarify possibility to extend SDK interfaces

[JonasKunz]

Split out from #4024 (see this comment).

I've attempted to merge the clarifications with the existing Extending API Calls section and tried to soften the wording to be less prescriptive.

Changes

Clarifies that it is permissible to extend SDK interface without warranting a new major version.

• Related issues #
• Related OTEP(s) #
• Links to the prototypes (when adding or changing features)
• CHANGELOG.md file updated for non-trivial changes
• spec-compliance-matrix.md updated if necessary

[tigrannajaryan]

I wrote a draft on this topic a while back that goes into a bit more details and nuances.

My be worth reviewing it and we can decide if we want to pursue it.

[JonasKunz]

@tigrannajaryan your draft indeed seems to cover my intent of this PR, but with a lot more detail.
I'd suggest that if we want to pursue having this in the spec, I'll close this PR in favour of a PR based on your draft opened by you?

[tigrannajaryan]

I'll close this PR in favour of a PR based on your draft opened by you?

@JonasKunz I won't be able to open a PR myself. If you'd like to continue working on this yourself please feel free to borrow any content from my doc.

[JonasKunz]

I'll close this PR in favour of a PR based on your draft opened by you?

@JonasKunz I won't be able to open a PR myself. If you'd like to continue working on this yourself please feel free to borrow any content from my doc.

@tigrannajaryan unfortunately I currently too don't have the capacity for this topic to that level of depth. I'll try to take some inspiration from your doc, but will keep the changes minimal and focused for the use case #4024. We can then later revisit with more details in follow-up PRs.

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 7 days.

[pellared]

Approving as I find it aligned with https://github.com/open-telemetry/opentelemetry-go/blob/main/CONTRIBUTING.md#how-to-change-other-interfaces.

@JonasKunz, thank you for your hard work ❤️