fix(distributor): validate partition ring is kafka is enabled

[cyriltovena]

What this PR does / why we need it:

I think it makes more sense to also enable partition ring when distributor push kafka is enabled

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

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• Documentation added
• Tests updated
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[benclive]

Approving but see the comment.

[benclive]

This makes sense but the distributor won't be able to send any data to Kafka unless the ingester is also enabled due to needing some active partitions. I wonder if we should just use a single flag to enable the kafka write path?

[cyriltovena]

I understand although it feels weird that having t.Cfg.Distributor.KafkaEnabled without t.Cfg.Ingester.KafkaIngestion.Enabled panics ? Should we validate instead ?

[benclive]

Yep, I think we need to validate that ingestion is enabled if distributor is enabled, since they're coupled in the architecture.

[benclive]

To clarify, I think we want to keep both flags because we want the option to stop writing to Kafka without changing the ingesters.
But we also want a validation for All (and maybe Write) mode to enforce that the ingester is enabled if the distributor is enabled. We don't want this check for micro-services mode though, since those flags can be set explicitly on those components.

[cyriltovena]

ok I'll add a validation. That will avoid the panic at least.