✨ Implement warm replica support for controllers

[godwinpang]

This change implements the proposal for warm replicas as proposed in #3121.
It adds an EnableWarmup option for controllers to optionally start as warmed replicas, which means that sources will be started before leader election has been won by the instance.

It also adds a new internal runnable interface called warmupRunnable with a single Warmup method that will be called before leader election. There is no guarantee that the Warmup method returns before leader election, just that it is called.

The controller's implementation of the Warmup method for the warmupRunnable starts the sources with a threadsafe method startEventSourcesAndQueueLocked that also adds events to the queue. In the case of a non-leader elected controller, it is intended for the Warmup method to race with the Start method to start the sources. The methods are synchronized by the didStartEventSourcesOnce sync.Once used in startEventSourcesAndQueueLocked.

For the most part, Warmup behaves exactly the same as Start with the following differences

• There is no restriction as to how many times Warmup can be called, vs. Start which can only be called once.
• Warmup doesn't initialize metrics for the controller, nor does it start the worker goroutines.

Shutdown

Warmup runnables are shutdown in a separate goroutine from the one that stops the leader election runnables, because there is no guarantee as to whether or not the Warmup or the Start method is holding the lock for the didStartEventSourcesOnce sync.Once instance.

Testing

On top of the unit + integration tests, this PR was tested with an internal project that ran this branch of controller-runtime for a week with (EnableWarmup=true`, and verified that the controllers on the non-leader elected replica had a populated workqueue.

[k8s-ci-robot]

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[sbueringer]

/ok-to-test

[godwinpang]

/retest

[godwinpang]

/retest

[godwinpang]

/retest

[godwinpang]

/retest

[godwinpang]

/retest

[godwinpang]

/retest

[godwinpang]

/retest

[sbueringer]

@godwinpang Thank you very much! We're almost there (only one idea for simplification in the prod code, otherwise just a few comments on tests)

[sbueringer]

@godwinpang would be nice if we could get this PR done 😃

[godwinpang]

@godwinpang would be nice if we could get this PR done 😃

Sorry for the long wait, I've addressed the PR comments

[sbueringer]

Thx, just some very minor findings now.

@alvaroaleman Can you please also take another look?

[godwinpang]

/retest

[sbueringer]

Thank you!

/lgtm

/assign @alvaroaleman Please take a final look, thx!

Once this PR merges, let's please update the proposal PR (#3121) and let's get it merged soon as well

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: ce10cc30548051ee6c7ae68d8944e2191f50891d

[alvaroaleman]

Thanks!
/hold cancel

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: alvaroaleman, godwinpang

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