Add queue depth metrics for indexer workers

[Chucks1093]

Summary
Add queue depth metrics for indexer workers

Scope

• emit queue depth metrics for indexer worker queues\n- include labels for queue type/state\n- document alerting thresholds

Acceptance Criteria

• scoped change is implemented with no unrelated file churn
• docs/tests are updated where applicable
• contributor can validate the outcome locally

Contributor coordination: if you pick this up, reach out in Telegram for updates or clarification: https://t.me/accesslayerorg

[caxtonacollins]

@caxtonacollins has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

Hi 👋 am a full-stack developer
I will love to work on this issue please assign

ℹ️ Repo Maintainers: To accept this application, review their application or assign @caxtonacollins to this issue.

[Uchechukwu-Ekezie]

@Uchechukwu-Ekezie has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

Please assign this to me

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Uchechukwu-Ekezie to this issue.

[drips-wave]

Congratulations, @Uchechukwu-Ekezie! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

🧑‍💻 @Uchechukwu-Ekezie: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

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[drips-wave]

This issue has been completed by @Uchechukwu-Ekezie as part of the Stellar Wave Program's 4th Wave 🥳

😎 @Uchechukwu-Ekezie: You earned 200 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here. You can also Leave a review to share your experience working on this issue.

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