PromQL: Allow label append to groups of metrics

[SuperQ]

Proposal

From an IRC conversation with @ecraven, we had an idea to simplify some expressions by allowing labels to be applied to groups of metrics surrounded by ().

For example:

foo{env="prod"} / bar{label="prod"}

Becomes:

(foo / bar){env="prod"}

The effect for other labels applied to foo and bar would be to append the list of label filters to any interior ones.

For example:

(foo{blah="blah"} / bar{blah="blah"}){env="prod"}

Would translate to:

foo{blah="blah",env="prod"} / bar{blah="blah",env="prod"}

[aixeshunter]

Good idea 👍

[brian-brazil]

I'm not sure about this. Given how the rest of the language works, I'd expect this to evaluate all of foo{blah="blah"} / bar{blah="blah"} and then restrict it down. I'm also generally against features that are merely another way of writing something that we already support.

[SuperQ]

Yes, I agree, it might over-complicate parsing. I just wanted to post the idea, as it seemed interesting and worth having a discussion about.

[ecraven]

The reason I tried this was a form like
((foo - bar - baaz) / foo) where I'd have to repeat the labels four times :-/