helm: add support for PMM

[derekperkins]

This adds optional support to auto-deploy PMM as a separate StatefulSet in Kubernetes, plus adds a pmm-client container to vttablet, along with 2 log tailing containers, which auto-register with the server. It enables mysql metrics and query analytics by default.

The PMM client requires root privileges in the container, so I was forced to remove runAsNonRoot from the default Pod Security Context, but all other containers are still running as a non-root user, so it shouldn't be an issue.

[derekperkins]

Also of note, I'm ignoring client errors until https://jira.percona.com/projects/PMM/issues/PMM-1985 is resolved. The client works, and worst case scenario where we ignore an error, it won't have any impact on actual Vitess/MySQL operations.

@enisoc This is ready for review

[enisoc]

We should put all these RUN commands into script file, and just COPY and RUN that. Currently since these are separate layers, we're still paying for the storage of all the removed files.

[derekperkins]

Ok, I can do that

[enisoc]

Do they not have an apt repo we can add?

[derekperkins]

They do. This seemed easier, but I can switch to their apt setup

[enisoc]

It seems weird to install a systemd service in a container. Can we not just run the server directly?

[derekperkins]

Their whole setup is very opinionated and there isn't a way around this. You have to use their pmm-admin client and it doesn't have an option to run another way.

[derekperkins]

I batched all the RUN commands together and it brought the image size from 72 MB down to 45 MB, so that was a win. Is there a reason using the apt repo is better than just grabbing the binary?

[enisoc]

LGTM