feat(o11y): add tokio metrics

[akundaz]

📝 Summary

Add tokio runtime and task metrics. The tasks monitored are:

• flashblock timer
• payload builder service
• payload handler
• websocket publisher

Tested locally with builder playground and lots of tokio metrics are being created.

> curl localhost:5555 | rg tokio | wc -l
409

💡 Motivation and Context

Resolves #240

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✅ I have completed the following steps:

• Run make lint
• Run make test
• Added tests (if applicable)