promql: sort_by_label should fallback to a consistent ordering by metric, not by sample value about prometheus HOT 5 OPEN

The function is still experimental, so we can change the behavior.

An additional bonus is that we don't have to look at the sample value at all anymore, which might allow some optimizations.

@juliusv @bboreham WDYT?

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I agree with your proposal

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@beorn7 I agree!

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I can't think how it can happen that two series have the same label value.
The PromQL engine checks between each operation and halts with an error if it finds this case.

However I agree it is a nice optimisation, to avoid doing work that cannot be useful.

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Oh, my mistake - it sorts by a subset of labels, which can have the same value.

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