Define an Entity Merge algorithm (from OTEP #264)

[jsuereth]

Changes

Adds the merge algorithm specified in OTEP#264 to the Resource + Entity data model specification (both still in development).

This merge algorithm has been used in Entity prototypes for the past year and has not seen any change. It's been adaptable enough to handle several re-architectures of SDKs through prototyping, so we're confident in pushing it forward into the specification.

A few caveats:

• See the note on Resource DataModel about conflicting merge algorithms. This one currently focused only on merging entities into resource when there is a known set of entities in the existing resource.
• I've split the merge algorithm into two: The Entity portion about eligibility of merging and descriptive attribute merging, and then the more robust resource section.

Link to prototypes:

• Java: https://github.com/jsuereth/opentelemetry-java/blob/wip-entity-and-providers/sdk/common/src/main/java/io/opentelemetry/sdk/entity/internal/EntityUtil.java#L289
• TypeScript: [WIP] entities prototype opentelemetry-js#5620

[github-actions]

This PR was marked stale. It will be closed in 14 days without additional activity.

[tigrannajaryan]

Do you see a possibility to relax the last requirement a bit? For example can we aim for: "schema_url is the same or schemas are compatible", where "schemas are compatible" if attributes used by Entities are unchanged between schema versions? I understand it is more work to check compatibility but it makes merging much more widely possible.

[jsuereth]

So in that event, we expect the following:

• The SDK/Collector has no way of knowing if a schema_url is compatible with another, without looking at the schema.
• If we have to look at the schema, we can convert the version succesffuly, and then we can merge two entities that are at the same version.

So while we do want to allow schema_url based conversions, the merge algorithm for Entities can be defined much more simply and we leave room for version-conversion prior to this.

[tigrannajaryan]

This is a fairly complicated algorithm. Can you please add a few examples to demonstrate what it is doing?

[jsuereth]

Added

[dyladan]

This has had a lot of discussion and several implementations with no complaints