return error at startup when no write path is configured

[areebahmeddd]

What this PR does:

• IngesterWritePathEnabled is hardcoded to false, and KafkaWritePathEnabled is set to ingest.Enabled (which defaults to false).
• With both flags off, the distributor starts without errors but silently drops all traces.
• This PR adds a check in Config.Validate() to throw a clear error if both write paths are disabled, so Tempo fails at startup with a useful error message.

Which issue(s) this PR fixes:

Fixes #6629

Checklist:

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

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[mdisibio]

Hi thank you for the fix.

@mapno @javiermolinar - Is this related to additional 3.0 cleanup needed? For example I'm thinking in 3.0 the distributor shouldn't be able to enter into this situation, it would always require the Kafka write-path to be enabled. But I remember we were considering a non-kafka solution for single binary.

[areebahmeddd]

I'm thinking in 3.0 the distributor shouldn't be able to enter into this situation, it would always require the Kafka write-path to be enabled.

yea i figured the same.

But I remember we were considering a non-kafka solution for single binary.

made a small change to the pr; switched the test target to fix the failing check. i randomly spent some time tracing this one down so hoping this gets merged 🙂