Added StartsWith and EndsWith Operators to Label Selectors

[CameronHall]

What type of PR is this?

/kind feature
/kind api-change

What this PR does / why we need it:

Sometimes we want to be able to select labels with values that “start with” or “end with” a particular string - I have a legitimate use case at the moment with external metrics. I understand that the recent proposition for regular expression support for selectors was rejected with concerns around performance.

In which case, I’m proposing an alternate solution that is more explicit and alleviates the potential performance impacts by bad actors or inefficient regular expressions.

Which issue(s) this PR fixes:

No specific issue yet - but it’s supplementary to #107053

Special notes for your reviewer:

I considered glob pattern support as suggested in #107053 but that’s a lot more effort and pushing the boundary of my Go/Kubernetes knowledge - I also much prefer the explicit nature of this approach with operators.

I have some questions for the team, around additional functionality.

• Should these operators be extended to field selectors as well?
• Should I add a string contains operator while I’m here?

Does this PR introduce a user-facing change?

Added new StartsWith `^=` and EndsWith `$=` operators to Label Selectors

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

Does this need a KEP?

* To select a label that starts with a particular string you may use the "StartsWith" operator denoted by `^=`
* To select a label that ends with a particular string you may use the "EndsWith" operator denoted by `$=`

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Cameron Hall (0e004a8847263b5f160afaf0c32f502346738c9d)

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: CameronHall
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[ncdc]

/uncc

[leilajal]

@CameronHall do we have a KEP for this? if yes, we need to discuss this in API Machinery SIG meeting.

[CameronHall]

@CameronHall do we have a KEP for this? if yes, we need to discuss this in API Machinery SIG meeting.

@leilajal it doesn't look like we have a KEP. Does it still have to be discussed in the SIG meeting?

[leilajal]

@CameronHall it seems for the changes that impact APIs we need a KEP.
You can still present this without KEP and see what the community suggests.

[MadhavJivrajani]

/ok-to-test

[fedebongio]

@CameronHall thanks, triaging for now - hoping to see you at the SIG meeting (or reach out to us with a KEP :) ).
/triage accepted

[CameronHall]

/retest

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@k8s-triage-robot: Closed this PR.

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