fix: protect ruler remote-write overrides map with a mutex when creating new appenders

[verejoel]

What this PR does / why we need it:
This fixes an additional data race identified when creating new appenders in a ruler handling many hundreds of rules.

Which issue(s) this PR fixes:
Fixes #11569

Special notes for your reviewer: It's not possible to defer the unlock in the Appender function, as we access the registry later when update the tenant configuration. Therefore we just put the guard around the singular operation when we access the remote write config.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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[verejoel]

@cstyan pinging you for a review :)

[cstyan]

@verejoel thanks! before I merge, can you confirm that you're just seeing this when you have many rules configured? it's not that you have few rules but multiple remote write endpoints for that ruler/tenant?

[verejoel]

Yes, we have around 800 rules configured. I was able to reduce the impact by scaling to 10 replicas (so around 80 rules per replica). But still see a crash about once every 30 minutes. The test case I wrote catches the failure.

…

On Mon, 29 Jul 2024 at 14:46, Callum Styan ***@***.***> wrote: @verejoel <https://github.com/verejoel> thanks! before I merge, can you confirm that you're just seeing this when you have many rules configured? it's not that you have few rules but multiple remote write endpoints for that ruler/tenant? — Reply to this email directly, view it on GitHub <#13676 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ALE5VOMOMVGJJNXQJ2TPQL3ZO3O6FAVCNFSM6AAAAABLQH6AKGVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDENJXGI2TIMJQGM> . You are receiving this because you were mentioned.Message ID: ***@***.***>

[cstyan]

@verejoel cool, thank you 👍