instructions to enable oldtimer API

[cainelli]

Hi there, I've added basic instructions on how to enable the old-timer API based on #1172. It took me some time to search on the repo. I think others may found useful to see this present on the proposal doc.

Thanks.

[k8s-ci-robot]

Thanks for your pull request. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please follow instructions at https://github.com/kubernetes/kubernetes/wiki/CLA-FAQ to sign the CLA.

Once you've signed, please reply here (e.g. "I signed it!") and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please sign in with your organization's credentials at https://identity.linuxfoundation.org/projects/cncf to be authorized.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.

[k8s-ci-robot]

Hi @cainelli. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with @k8s-bot ok to test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.

[k8s-reviewable]

This change is 

[cainelli]

I signed it!

[DirectXMan12]

Oldtimer was never widely implemented, and is very likely going to be deprecated and removed over the next couple of releases. I'm not entirely certain it's worth providing these instructions (especially without a deprecation notice).

@piosz WDYT

[cainelli]

So will the main endpoints extract metrics older than 15 min. or if we want to do that we'll have to query the backend sink?

[DirectXMan12]

So will the main endpoints extract metrics older than 15 min. or if we want to do that we'll have to query the backend sink?

Eventually we'll have something along the same lines as the new custom metrics API -- a set of type definitions and API paths that will be implemented separately for each sink, except in a clearly historical context (as opposed to the custom metrics API, which is for recent metrics).

[AlmogBaku]

Oldtimer was misspelled

[piosz]

#1577 (comment)
+1