Ensure binary operation is not mapped if operands are not split

[ssncferreira]

What this PR does

Ensure binary operation is not mapped if operands are not split. This is done by checking the stats.GetShardedQueries() metric.

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]

[pracucci]

LGTM (modulo a couple of nits)

[pracucci]

Question: what are the implications if only 1 of the 2 legs have finished? What does it mean on the other one?

[ssncferreira]

I think no implications, one of the legs will be split and the other not. I assume the sharding middleware will then do the sharding on each leg separately.
There are also tests to binary operations with these cases:

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

Lines 118 to 133 in 049ad98

	// Binary operations
	{
	in: `rate({app="foo"}[3m]) / rate({app="baz"}[6m])`,
	out: `(sum without() (` + concatOffsets(splitInterval, 3, `increase({app="foo"}[x]y)`) + `) / 180) / (sum without() (` + concatOffsets(splitInterval, 6, `increase({app="baz"}[x]y)`) + `) / 360)`,
	expectedSplitQueries: 9,
	},
	{
	in: `rate({app="foo"}[3m]) / 10`,
	out: `(sum without() (` + concatOffsets(splitInterval, 3, `increase({app="foo"}[x]y)`) + `) / 180) / (10)`,
	expectedSplitQueries: 3,
	},
	{
	in: `10 / rate({app="foo"}[3m])`,
	out: `(10) / (sum without() (` + concatOffsets(splitInterval, 3, `increase({app="foo"}[x]y)`) + `) / 180)`,
	expectedSplitQueries: 3,
	},