chore(source/crd): optimize endpoint labels without looping over

[ivankatliarchuk]

What does it do ?

Relates #5243

Motivation

Working with CRD, I found that labels applied in a loop, such as ResourceLabelKey. The flow looks like that

flowchart TD
    A[Start Endpoints Method] --> B[List DNSEndpoints from Informer]
    B --> C{Error?}
    C -- Yes --> Z[Return Error]
    C -- No --> D[Loop over DNSEndpoint Items]
    D --> E[Extract Metadata and TTL]
    E --> F[Loop over Spec.Endpoints]
    F --> G[Convert to endpoint.Endpoint]
    G --> H[Call setResourceLabel]
    H --> I[Loop over Labels to set resource label]
    I --> J[Append to Final Result List]
    J --> K{More Spec.Endpoints?}
    K -- Yes --> F
    K -- No --> L{More DNSEndpoints?}
    L -- Yes --> D
    L -- No --> M[Return Final Result List]

Loading

Let:

• n = number of DNSEndpoint resources
• m = average number of .Spec.Endpoints per resource
• k = average number of Labels in each resource

Then:

• Outer loop: n

• Inner loop: m (per .Spec.Endpoints)

• Nested call: setResourceLabel includes another loop of k

• Overall time complexity:
  O(n * m + n * k)

With this tweak, reducing time complexity to O(n * m). Not a massive gain, but still.

follow-up:

• review other sources, seems like not every source is setting ResourceLabelKey or setting them in a loop.
• other sources not testing where or not ResourceLabelKey is set for generated endpoints

More

• Yes, this PR title follows Conventional Commits
• Yes, I added unit tests
• Yes, I updated end user documentation accordingly

switch to more efficient string slicer

[mloiseleur]

/lgtm
/approve

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