refactor: controller manager: InitFunc and base controller interface.

[jiahuif]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR adds an interface for all controllers, as defined as follows:

// Interface defines the base of a controller managed by a controller manager
type Interface interface {
}

// Debuggable defines a controller that requests the controller manager
// to expose a debugging handler for the controller
//
// If a controller implements Debuggable, and the returned handler is
// not nil, the controller manager will mount the handler during startup.
type Debuggable interface {
	// DebuggingHandler returns a Handler that expose debugging information
	// for the controller, or nil if a debugging handler is not desired.
	//
	// The handler will be accessible at "/debug/controllers/{controllerName}/".
	DebuggingHandler() http.Handler
}

Originally, a controller expose debugging handler by returning the handler through InitFunc, added in #66623

// InitFunc is used to launch a particular controller.  It may run additional "should I activate checks".
// Any error returned will cause the controller process to `Fatal`
// The bool indicates whether the controller was enabled.
type InitFunc func(ctx ControllerContext) (debuggingHandler http.Handler, enabled bool, err error)

This method limits the extensibility of controllers. If we need to add other features (e.g. health checks) to each controller, we have to yet add another return value. Also, it will benefit us to have a standardized controller.Interface type.

Which issue(s) this PR fixes:

Special notes for your reviewer:
cloud-controller-manager explicitly disallowed debuggingHandler.

https://github.com/kubernetes/kubernetes/blob/v1.22.1/staging/src/k8s.io/cloud-provider/app/controllermanager.go#L276

Does this PR introduce a user-facing change?:

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

https://kubernetes.io/docs/concepts/architecture/controller/
https://github.com/kubernetes/kubernetes/pull/66623

[jiahuif]

/assign @cheftako

[fedebongio]

/triage accepted

[k8s-triage-robot]

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/lifecycle stale

[jiahuif]

/remove-lifecycle stale
because of a split PR.

[cheftako]

Looks like format need to be run.

[jiahuif]

Rebased this PR as go is upgraded to 1.17

[cheftako]

If we want GarbageCollector to implement the controller.Interface then we should add a check when we define GarbageCollector that it implements that interface.

var _ contoller.Interface = GarbageCollector{}

[jiahuif]

Thanks. Added these assertions below the def. of GarbageCollector.

[cheftako]

/lgtm
/approve

[jiahuif]

/assign @BenTheElder

could you take a look at vendor/?

[BenTheElder]

/approve
for vendor/modules.txt

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: BenTheElder, cheftako, jiahuif

The full list of commands accepted by this bot can be found here.

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Needs approval from an approver in each of these files:

• cmd/cloud-controller-manager/OWNERS [cheftako]
• cmd/kube-controller-manager/OWNERS [cheftako]
• pkg/controller/garbagecollector/OWNERS [cheftako]
• staging/src/k8s.io/cloud-provider/OWNERS [cheftako]
• vendor/OWNERS [BenTheElder]

Approvers can indicate their approval by writing /approve in a comment
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[k8s-ci-robot]

@jiahuif: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Rerun command
pull-kubernetes-conformance-kind-ipv6-parallel	ceaab9c70b6be01bf797c38e296356ba8aca77ee	link	/test pull-kubernetes-conformance-kind-ipv6-parallel
pull-kubernetes-e2e-ubuntu-gce-network-policies	ceaab9c70b6be01bf797c38e296356ba8aca77ee	link	/test pull-kubernetes-e2e-ubuntu-gce-network-policies
pull-kubernetes-conformance-image-test	ceaab9c70b6be01bf797c38e296356ba8aca77ee	link	/test pull-kubernetes-conformance-image-test
pull-kubernetes-e2e-gce-alpha-features	ceaab9c70b6be01bf797c38e296356ba8aca77ee	link	/test pull-kubernetes-e2e-gce-alpha-features

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[jiahuif]

/retest