Add External Type Checking

[hdost]

The purpose of this is to ensure we understand the API surface area. Anything exposed by a crate like a type can affect the possible compatibility. This means that if we bump an external type it could cause breaking changes. This tracks them to limit this possibility.

Affected Crates:

• opentelemetry-otlp
• opentelemetry-zipkin

Fixes # Design discussion issue (if applicable) #

Changes

Please provide a brief description of the changes here.

Merge requirement checklist

• [ ] CONTRIBUTING guidelines followed
• [ ] Unit tests added/updated (if applicable)
• [ ] Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• [ ] Changes in public API reviewed (if applicable)