Optimize Registration Logic for Snapshot Compaction controller

[seshachalam-yv]

How to categorize this PR?
/area storage
/kind enhancement

What this PR does / why we need it:
This PR optimizes the logic behind registering snapshot compaction-controller. Instead of always running the compaction-controller and checking the CLI flag --enable-backup-compaction just before deploying the compaction job, the new approach registers the compaction-controller based on the presence of the CLI flag --enable-backup-compaction.

Which issue(s) this PR fixes:
Fixes #655 (comment)

Special notes for your reviewer:

Release note:

None

[seshachalam-yv]

/test pull-etcd-druid-e2e-kind-alpha-features

[unmarshall]

With these changes, the controller will only run if either EnableSnapshotCompaction feature gate or the CLI flag --enable-backup-compaction is enabled. A TODO comment has been added to highlight the planned removal of the CLI flag.

Taking an example of KAPI feature-gates, you can see that once the feature is GA then it is always enabled and cannot be disabled. This is how feature-gates are used in several k8s components. A CLI flag for the same has a different purpose and that is to enable or disable a feature/functionality irrespective of its state (alpha/beta/GA).

Q: What is our interpretation of feature gates? Is it in line with all upstream k8s components or is it used to disable/enable features/functionality irrespective of its state(alpha/beta/GA)? The reason i asked this question is that you have put this feature gate along with etcd-wrapper feature gate. Once etcd-wrapper feature gate is GA, then it will always be used and there will not be any option to disable it.

Also have a look at Feature-Gates-Removed where a lot of GA feature gates have been removed as for GA features, consumers cannot influence the decision to include or exclude them.

My recommendation is to have only a single interpretation of feature gates. One cannot have both interpretations. For example for etcd-events there are no snapshot leases and therefore you would always want to disable compaction controller for etcd-events. From this perspective it is not really a feature gate (if you interpret it the way upstream k8s does it and also the way it has been done for etcd-wrapper).

[seshachalam-yv]

/test all

[shreyas-s-rao]

@seshachalam-yv thanks for making the required changes.
/lgtm

[abdasgupta]

Thanks for the PR. LGTM