Remove upfront check whether an instant query is splittable or not

[pracucci]

What this PR does

Currently we need to change logic in 2 places when we add support to split more type of queries: both isSplittable() function and the actual instantSplitter logic.

I want to remove isSplittable() and let go every single query through the mapping logic. At the end of the mapping, we already check if it was successfully split or not, to decide whether it will be executed by the query-frontend (if was split) or just fully run it downstream (if wasn't split).

Which issue(s) this PR fixes or relates to

N/A

Checklist

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

[ssncferreira]

LGTM 🎬
Just a comment about the impact of this change

[ssncferreira]

I'm not sure if this will introduce problems 🤔 (need to think a bit more about this).
But by removing this we are opening the door to all expressions wrapped by vector aggregators to be split.
Maybe it won't be a problem because we only send downstream the splittableVectorAggregators vectors 🕵️‍♀️

[pracucci]

Ok, let's investigate if this could cause issues. If it does, then we should port this logic to instantSplitter.mapAggregatorExpr().

In instantSplitter.mapAggregatorExpr() we have:

mimir/pkg/frontend/querymiddleware/astmapper/instant_splitting.go

Lines 149 to 153 in cc0ef42

	if i.outerAggregationExpr == nil && splittableVectorAggregators[expr.Op] {
	mappedExpr, finished, err = NewASTExprMapper(i.copyWithEmbeddedExpr(expr)).MapExpr(expr.Expr, stats)
	} else {
	mappedExpr, finished, err = i.MapExpr(expr.Expr, stats)
	}

Since splittableVectorAggregators is already stricter, the risk of this change could from here:

mimir/pkg/frontend/querymiddleware/astmapper/instant_splitting.go

Line 152 in cc0ef42

mappedExpr, finished, err = i.MapExpr(expr.Expr, stats)

[colega]

As long as the tests pass, this should be fine 🤷

[ssncferreira]

LGTM