MQE: Add support for round

[jhesketh]

What this PR does

Which issue(s) this PR fixes or relates to

Fixes #

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• about-versioning.md updated with experimental features.

[jhesketh]

(note that I'll update the point indexes in a separate change because I want discuss an implementation specific)

[charleskorn]

Does not consistently drop the __name__ label, or doesn't drop it at all?

Is there an issue link for this?

[jhesketh]

This is tricky. Against main, round does not return any name (as evidence by the tests here prometheus/prometheus#15176).

Against what we have vendored it does have the name. So the bug has seemingly since been fixed. I have not found an exact issue but assume it is related to the more recent delayed label name dropping work fixing something incidentally.

Also worth noting is that in the vendored version it has DropName: true,, so I'm not sure if that isn't being honoured somewhere (I haven't tracked down where).

The only upstream tests of round are in the aggregations.test file, such as:

eval instant at 50m round(0.005 * http_requests{group="production",job="api-server"})
	{group="production", instance="0", job="api-server"} 1
	{group="production", instance="1", job="api-server"} 1

Which due to the multiplication has the resulting label dropped.

So it wouldn't surprise me if it went unnoticed upstream and also fixed as part of the label work (also unnoticed).

[charleskorn]

Right, thanks for the explanation.

Also worth noting is that in the vendored version it has DropName: true,, so I'm not sure if that isn't being honoured somewhere (I haven't tracked down where).

DropName is only used when the experimental new delayed __name__ dropping is enabled, and this is disabled by default.

I would rephrase the comment slightly to make it clearer, something like this perhaps:

Suggested change

	// TODO(jhesketh): With the currently vendored prometheus, round does not consistently drop the __name__ label
	// TODO(jhesketh): With the version of Prometheus vendored at the time of writing, round does not drop the __name__ label, and this is verified by our tests.
	// We match this behaviour for consistency, but this behaviour has changed in Prometheus 3.0, so we'll need to match that once Prometheus 3.0 is vendored in.