Consider adding optional fields and functions in Interfaces to avoid breaking changes in minor version updates

[sgohlke]

We use opentelemetry-js in some of our projects and have a small implementation of the Span interface to be used in our unit tests. With the release of api v1.9 the Span interface contains additional non-optional fields/functions addLink and addLinks: #4528 . This broke our implementation of the interface.

While I can part of understand the reasoning stated in https://github.com/open-telemetry/opentelemetry-specification/blob/a03382ada8afa9415266a84dafac0510ec8c160f/specification/upgrading.md?plain=1#L97-L122 I would like to point out that it is possible to define optional fields/functions in interfaces. This can e.g. be used to add new logic in minor versions without breaking existing implementations and set the field/function to required in the next major version. This might come with a small drawback of a check like 'addLink' in span to ensure the function exists before calling it.

• This only affects the JavaScript OpenTelemetry library
• This may affect other libraries, but I would like to get opinions here first

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