🌱 Introduce a new runnable group for basic servers of the manager

[zqzten]

This PR introduces a new runnable group for basic servers (health probes, metrics and pprof server) of the manager and make it start before any other runnables. This can resolve the cyclic dependency issue after we changing these servers to runnables.

[zqzten]

cc @sbueringer plz double check if this works under your case, thanks

[sbueringer]

I'll take a look soon, but I would delay merging it until after the v0.15 release (just because we don't strictly need it ASAP + it is a risk directly before the release)

[vincepri]

Is there a specific problem we're trying to solve to have the servers in a runnable group?

[zqzten]

Is there a specific problem we're trying to solve to have the servers in a runnable group?

Yep, if we don't have them in a specific runnable group, they would be added to the Others runnable group by default, which would be started after the caches are ready. This can lead to this cyclic dependency issue.

In a word, we need to have the servers in a specific runnable group which would be started first (at least before the Caches runnable group) in order to keep the same start order as which before they become Runnables.

[sbueringer]

I think Vince might be more asking why should it be a runnable vs the current state.

To answer that, @vincepri this goes back to #1943 (comment)

[sbueringer]

Took me a while to find some time to work on the test. But there's now a PR up to ensure we don't hit deadlocks during manager.Start: #2418

We should probably merge #2418 before this PR

[k8s-ci-robot]

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[sbueringer]

/lgtm