Clarify Attributes advisory parameter

[pichlermarc]

Fixes #4037

Changes

Clarifies that Attributes is an allow-list of metric keys.

• Related issues Clarify Attributes instrument advisory parameter  #4037, [metrics] prototype attributes instrument advisory parameter opentelemetry-js#4365

[jsuereth]

First - MAY is normative and should be capitalized when used.

Second - I'm not sure this improves the understanding of what will happen here.

It is is true that this set of attribute keys MAY be used by the SDK, and only attributes found in this set will "pass" through if this set is used. However "allowed" implied that we'd erase measurements that contain other attribute keys, which isn't true.

In the SDK portion of this, we explicitly state that other attribtutes are ignored when using the term "allow list".

I'd suggest we use very similar terminology to the SDK here:

 This is an allow-list of attribute keys for measurements captured in the metric stream that MAY be used. When used, the allow-list contains attribute keys that identify the attributes that MUST be kept, and all other attributes MUST be ignored.

Possibly we could link to the SDK here, since I'm not sure we've introduced the term "metric stream" yet.

[pichlermarc]

Yep that makes sense. I applied the suggestion in 9672d5b and linked the same data model subsection as the SDK does when it introduces the concept of a metrics stream.

[github-actions]

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

[cijothomas]

that MAY be used. When used,

It is not clear what does "use" means here?

The SDK side of the spec also feels a bit odd to me..
Is the Attributes advisory parameter used by default in SDK?
or user must explicitly opt-in to say "take attributes from attributes advisory, if any. else take everything"?
(This maybe outside of the this PR's scope, but I think clarifying the SDK behavior is important before tweaking the wording here...)

[trask]

(this is tricky, also not sure MUST is correct since it's just "advice")

Suggested change

	`Attributes` (a list of [attribute keys](../common/README.md#attribute)) is an
	allow-list of attribute keys for measurements captured in the [metric stream](../metrics/data-model.md#events--data-stream--timeseries)
	that MAY be used. When used, the allow-list contains attribute keys that
	identify the attributes that MUST be kept, and all other attributes MUST be
	ignored.
	`Attributes` (a list of [attribute keys](../common/README.md#attribute)) is
	the recommended set of attribute keys to be retained in the resulting metrics.

[github-actions]

Closed as inactive. Feel free to reopen if this PR is still being worked on.