Add supplementary guidelines for SDK self-observability

[cijothomas]

Adds non-normative supplementary guidelines covering lifecycle ordering, avoiding telemetry-induced-telemetry loops, and stability handling for SDK self-observability.

Towards #5128. Follow-up to #5087.

[dashpole]

LGTM.

[Copilot]

Pull request overview

Adds a new non-normative “supplementary guidelines” document to expand the SDK self-observability guidance introduced in self-observability.md, and wires it into the existing docs and changelog.

Changes:

• Add specification/self-observability-supplementary-guidelines.md with implementation guidance on lifecycle ordering, avoiding telemetry loops, and stability handling.
• Link the supplementary guidelines from specification/self-observability.md.
• Add an Unreleased changelog entry for the new supplementary guidelines document.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
specification/self-observability.md	Adds a pointer to the new supplementary guidelines document.
specification/self-observability-supplementary-guidelines.md	Introduces the new non-normative guidance document for SDK implementers.
CHANGELOG.md	Records the addition under “Supplementary Guidelines” in Unreleased.

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[pellared]

nit: can we just use "events" consistently?

[pellared]

Issue #5128 asked for a few topics this PR does not really cover: startup configuration event/log and schema URL / telemetry versioning guidance. Since the PR says "Fixes #5128", I’d either add short sections for those, or explicitly say they are deferred/follow-up. We can also change to "Towards #5128". Otherwise the doc is good, but the issue closure may feel a little broader than the actual content.

It would be probably good to describe something like a "lazy provider" in a separate PR.