Move metrics SDK spec to Mixed

[reyang]

The Metrics SDK specification stabilization progress is tracked by https://github.com/orgs/open-telemetry/projects/9.

During the past 4 weeks, we haven't seen outstanding new issues during the weekly triage meeting, and all of the known blocking issues have been properly resolved.

Changes

• Mark Metrics SDK specification to Mixed, with Exemplar as Feature-freeze while other parts as Stable.

[github-actions]

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

[reyang]

We should revisit this PR once #2317 is merged.

[github-actions]

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

[jmacd]

Note that #2072 appears to be still blocking this PR, unfortunately. In #2072 (comment) I have a proposal that would also let us close #2232, I think. Please give #2072 and #2232 your attention, especially @mateuszrzeszutek, @legendecas, @jack-berg, @jsuereth, @reyang thanks.

[jack-berg]

I think that having some sort of resolution of #2314 is important for stabilization. My read is that there's consensus that delta backends actually want cumulative non-monotonic sums instead of delta non-monotonic sums for sync / async up down counter instruments. Stabilizing without some resolution would create trouble for these backends without benefiting anyone (as far as I can tell).

[github-actions]

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

[cijothomas]

re-approval after more recent PRs are merged.

[reyang]

This PR is blocked by a CI issue which is being addressed in #2426.

[mtwo]

Following up from the specification call today: this is ready to merge