feat: Revisit OTEL & setup Grafana + Prometheus + metrics dashboard

[alexluong]

I noticed Uptrace metrics were a bit inconsistent with missing data points sometimes. I'm not 100% sure why. I tried Prometheus and things seem to be a lot more correct. With that in mind, I set up Grafana & Prometheus with a super simple dashboard (not production-ready but could be a good starting point) to showcase our existing metrics.

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