Allow empty matcher at store gateway to fetch all postings

[yeya24]

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

If a query only contains external labels, no matcher is left after matching external labels at store gateway and in this case store gateway fetches nothing. https://github.com/thanos-io/thanos/blob/main/pkg/store/bucket.go#L2471

This PR changes Store Gateway to add a matcher ""="" to fetch all postings instead. This should match Prometheus behavior better.

This changes existing behavior. Current behavior all Stores will return nil. After that, Store Gateway will try to fetch all postings (series). This change is more compatible to Prometheus from what I understand. For example, cluster is just a normal label in Prometheus and users can do count ({cluster="A"}) without issues. However, after they make cluster an external label, then there is no way to run count ({cluster="A"}) anymore.

If there is concern about the behavior change, I can put this behavior under a FF and make it hidden to Thanos users.

Verification

Updated E2E test case.

[GiedriusS]

We have this check right now to forbid this kind of situations: https://github.com/search?q=repo%3Athanos-io%2Fthanos+%22no+matchers+specified%22&type=code. What's the relationship between those checks and your change? Is this something you need in Cortex?

[yeya24]

@GiedriusS Yes, we are evaluating external label based sharding in Cortex where blocks can have external labels. Similar to #7256.
Without this PR, a query with a single external label matcher only will break and returns no data. Imagine the migration from no external label blocks to external label blocks.

I understand that this changes the behavior of Store Gateway. Does it sound ok if we put this under a FF?