metrics-addr is not experimental anymore since 20.10

[u1735067]

• relates to Do not require "experimental" for metrics API moby/moby#40427
• relates to metrics: prometheus integration road map moby/moby#27307

- What I did
Removed obsolete phrases about experimental aspect of metrics-addr, see moby/moby@f337a8d

- How to verify it
Preview the page docs/reference/commandline/dockerd.md of this commit

- Description for the changelog
Fix doc mentioning metrics-addr as still experimental

- A picture of a cute animal (not mandatory but encouraged)
https://twitter.com/TweetsOfCats/status/1674110876863823891

[vvoland]

Thank you for contributing! It appears your commit message is missing a DCO sign-off,
causing the DCO check to fail.

We require all commit messages to have a Signed-off-by line with your name
and e-mail (see "Sign your work"
in the CONTRIBUTING.md in this repository), which looks something like:

Signed-off-by: YourFirsName YourLastName <yourname@example.org>

There is no need to open a new pull request, but to fix this (and make CI pass),
you need to amend the commit(s) in this pull request, and "force push" the amended
commit.

Unfortunately, it's not possible to do so through GitHub's web UI, so this needs
to be done through the git commandline.

You can find some instructions in the output of the DCO check (which can be found
in the "checks" tab on this pull request), as well as in the Moby contributing guide.

Steps to do so "roughly" come down to:

1. Set your name and e-mail in git's configuration:

   git config --global user.name "YourFirstName YourLastName"
   git config --global user.email "yourname@example.org"

   (Make sure to use your real name (not your GitHub username/handle) and e-mail)

2. Clone your fork locally

3. Check out the branch associated with this pull request

4. Sign-off and amend the existing commit(s)

   git commit --amend --no-edit --signoff

   If your pull request contains multiple commits, either squash the commits (if
   needed) or sign-off each individual commit.

5. Force push your branch to GitHub (using the --force or --force-with-lease flags) to update the pull request.

Sorry for the hassle (I wish GitHub would make this a bit easier to do), and let me know if you need help or more detailed instructions!

[u1735067]

Thank you for contributing! It appears your commit message is missing a DCO sign-off, causing the DCO check to fail.

Unfortunately, it's not possible to do so through GitHub's web UI, so this needs to be done through the git commandline.

[..]

Sorry for the hassle (I wish GitHub would make this a bit easier to do), and let me know if you need help or more detailed instructions!

You're welcome ; sorry about that, I'm currently only able to use the WebUI and I probably won't be able to do the sign-off (and squash the commits) until a few weeks. Since this is really a small change, feel free to take this change under your name if it can help 😊

[u1735067]

I don't know if this message is required, but the sign-off and squash have been done :)

[thaJeztah]

We may want to keep some wording about this.

The TL;DR is that we removed the experimental gate on the daemon side for the metrics socket, because we saw no strong motivation to require both the experimental option to be set, and setting the option for the metrics-address.

That said, naming of metrics may still change (also see moby/moby#27307); an initial set of metrics implemented, and the original plan was to add / change those based on feedback (we didn't get a lot of that, and I don't think here's been much activity around it).

[dvdksn]

I'd say fine to remove this. The docs shouldn't discuss things that may or may not happen in the future.