check for stream selectors to have atleast one equality matcher

[sandeepsukhani]

What this PR does / why we need it:
To avoid users doing a query of death by selecting all the streams or very broad queries we want to make users have at least one equality matcher in the selectors.

Special notes for your reviewer:
We also consider foo=~"bar|baz" as an equality matcher since it is internally converted to equality matchers.

Checklist

• Tests updated

[owen-d]

The code looks good, but I'm not sure about our goal here.

For instance, this seems a valid query, but we'd limit it: {job=~"loki-prod.*/querier"}.

What if we instead limited queries which had no matchers, such as returned by the cortex function SplitFiltersAndMatchers?

That would disallow queries such as {job=~".*"} but allow ones like {job=~".+"}

[sandeepsukhani]

The code looks good, but I'm not sure about our goal here.

For instance, this seems a valid query, but we'd limit it: {job=~"loki-prod.*/querier"}.

What if we instead limited queries which had no matchers, such as returned by the cortex function SplitFiltersAndMatchers?

That would disallow queries such as {job=~".*"} but allow ones like {job=~".+"}

Thats interesting. This would mean we would only allow matchers which would not make us fetch all the chunks for the whole query duration. I am not against this but I think it would be hard to explain it to the users because {job="foo"} would work but {job=""} will not.

[owen-d]

For an error message, what about: Queries require at least one regexp or equality matcher that does not have an empty-compatible value. For instance, app=~".*" does not meet this requirement, but app=~".+" will.

[stale]

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