test: Use interface for passing instance ring to partition instance ring

[benclive]

What this PR does:

Define a InstanceRingReader interface for creating a new PartitionInstanceRing.

• This is to enable mocking the Partition Instance Ring during tests external to this package.
• Specifically, Loki already uses an ingester ring mock to test certain functionality, such as looking up ingesters for querying, without defining a full, in-memory ring. I would like to extend this test to use a partition instance ring without rewriting the whole test suite, but this isn't possible right now.
• I cannot use approaches such as &Ring{ringDesc: &Desc{}} outside of the ring package because ringDesc is private, and the existing ReadRing interface does not provide the GetInstance(string) method.

Which issue(s) this PR fixes:
n/a

Checklist

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

[CLAassistant]

All committers have signed the CLA.

[pstibrany]

This change seems unnecessary to me. You can as well just keep reference to the ring provided to NewPartitionInstanceRing function, without introducing new interface.

[benclive]

keep reference to the ring provided to NewPartitionInstanceRing function

The problem I want to solve, which isn't present in this repo, is to be able to provide a mocked ring implementation to NewPartitionInstanceRing so I can test external behaviour that depends on the it without initialising a full Ring, KVStore & running service.

Specifically, in Loki I'm looking to provide a static list of ingesters & partitions in tests so I can assert that the query engine generates the correct behaviour in various scenarios. I can't easily do that right now because I need to provide a real Ring implementation to the constructor.

I'm aware it is possible to set up the real implementation for a test but it requires a lot of setup that isn't related to what I actually want to test, so adding an interface here seems simpler, quicker and less error prone approach to testing component behaviour.

[pstibrany]

I see that my idea doesn't solve your problem. I've checked the Mimir code and we don't call partitionRing.InstanceRing() method, approved.