[Epic] Implement observability with OpenTelemetry

[callahad]

As an application server, Unit is well placed to provide automatic tracing / metrics / etc. just by virtue of hosting runtimes. We should do that.

For example, Caddy (and thus FrankenPHP) have dead simple support for Prometheus metrics: https://caddyserver.com/docs/metrics. We should be at least as easy to enable.

Potential Tasks:

• [x] #1283
• [ ] #1284
• [ ] #1285
• [ ] #1286
• [ ] #837