Mark metrics as "alpha" or "stable"

[lbernick]

As part of our V1 planning, we will follow the Kubernetes deprecation policy, which specifies different deprecation policies for metrics marked as "alpha" and metrics marked as "stable". ("alpha" metrics are for use by developers of the project, while "stable" metrics are for use by users of the project.) We should go through Tekton's metrics and determine which are meant primarily for Tekton developers vs Tekton users, and mark the stability levels as appropriate.

We should also determine whether this information should be built into the Tekton implementation in some way or just added to the documentation.

[tekton-robot]

Issues go stale after 90d of inactivity.
Mark the issue as fresh with /remove-lifecycle stale with a justification.
Stale issues rot after an additional 30d of inactivity and eventually close.
If this issue is safe to close now please do so with /close with a justification.
If this issue should be exempted, mark the issue as frozen with /lifecycle frozen with a justification.

/lifecycle stale

Send feedback to tektoncd/plumbing.

[tekton-robot]

Stale issues rot after 30d of inactivity.
Mark the issue as fresh with /remove-lifecycle rotten with a justification.
Rotten issues close after an additional 30d of inactivity.
If this issue is safe to close now please do so with /close with a justification.
If this issue should be exempted, mark the issue as frozen with /lifecycle frozen with a justification.

/lifecycle rotten

Send feedback to tektoncd/plumbing.

[vdemeester]

/remove-lifecycle rotten
/lifecycle frozen