Allow cloud.* ad faas.* resource attributes to be set on span

[mateuszrzeszutek]

Fixes #1261

Changes

Add a note to faas and cloud resource attributes that allows them to be used as span attributes in serverless functions.

[Oberon00]

Nit: Use "Note:" only for explanations, not for specifying/allowing additional things.

[mateuszrzeszutek]

@open-telemetry/technical-committee @open-telemetry/specs-approvers Please take a look at this PR when you get a minute.

[anuraaga]

We might consider a general statement at the resource level that any attribute that cannot be determined at startup time can be added as span attributes.

Even better might be wording that it's ok for a resource to be mutable or dynamic. I think we assume it's constant, but I think it limits some use cases for attributes (docker cgroup CPU limit?).

But this still seems like a fine step to me.

[carlosalberto]

Overall good, but I'd like to get more eyes before we merge as I feel this change could be needed in other components too (I will add it to the agenda of the next SIG meeting, in case we haven't resolved this issue/PR by then).

[github-actions]

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

[Oberon00]

Blocking merge for now, I think it would be best to discuss the feasibility of #1298 first.

[github-actions]

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