aggregateutil: allow filtering against empty attribute set

[braydonk]

Description:
It is a valid use case to aggregate against an empty label set, which will functionally clear all attributes. This behaviour was removed in #33655, which simplified the check to len() == 0, which covers the case of the label set being nil and having 0 elements as the same scenario. However, these are not the same scenario and have different meanings. This PR reintroduces the original behaviour, but in a more efficient way by recognizing a label set with 0 elements and clearing the attributes, which would be the logical conclusion after running the filter anyway.

Link to tracking Issue: #34430

Testing:

Documentation:

[braydonk]

CC @odubajDT

[braydonk]

Integration test looks like a flake to me. I don't have authority to re-run the CI with the button but I can force-push if I need to re-run.

[odubajDT]

cc @evan-bradley @TylerHelmuth

[odubajDT]

please run make genotelcontribcol and commit the changes in this PR. Otherwise this change makes sense to me, if the codeowners agree with it

[braydonk]

Is there any chance this PR can be reviewed?

[TylerHelmuth]

@braydonk to make sure the functionality is the same as before can you also restore any tests to the metricstransformprocessor that covers this scenario.

[braydonk]

This is done in f7c46ce. Thanks for calling it out, I should have thought of that and it did expose a problem that I fixed in that commit as well.

While manually testing this along with the unit tests, I found something interesting. While in the previous version of the filterAttrs function acted differently if the filterAttrKeys argument was nil, metricstransform actually would never pass nil through that far. In the config building code, nil and [] would both end up resulting in [] thanks to this code path.

In f7c46ce I adjusted it to check for nil explicitly, allowing these cases to be treated differently. Should I keep this completely with the old behaviour? (i.e. not specifying label_set and explicitly setting label_set: [] both having the same result)

[TylerHelmuth]

Should I keep this completely with the old behaviour? (i.e. not specifying label_set and explicitly setting label_set: [] both having the same result)

Ya lets restore the previous behavior, it doesn't sound like the way metricstransform was doing this before was a bug.

[braydonk]

Fine by me. Having a nil label_set is a weird use of that operation anyway.

Restored in debfca5