feat: Add Prometheus metrics endpoint

[dbumblis-parabol]

Description

Fixes/Partially Fixes (https://github.com/ParabolInc/action-devops/issues/262)

• Adds prom-client package
• Metrics handler
• Adds metrics server if env var is set
• Exports websocket metrics

Demo

https://www.loom.com/share/9aa4f099360d46479f4ab3129041c261

Testing scenarios

• Scenario A (testing locally)
  • Set ENABLE_METRICS='true' & METRICS_PORT=9090 in env file
  • Observe the metrics server listen on specified port (or default port of 9090)
  • Visit localhost:9090/metrics and see exported values in prometheus format

Final checklist

• I checked the code review guidelines
• I have added Metrics Representative as reviewer(s) if my PR invovles metrics/data/analytics related changes
• I have performed a self-review of my code, the same way I'd do it for any other team member
• I have tested all cases I listed in the testing scenarios and I haven't found any issues or regressions
• Whenever I took a non-obvious choice I added a comment explaining why I did it this way
• I added the label Skip Maintainer Review Indicating the PR only requires reviewer review and can be merged right after it's approved if the PR introduces only minor changes, does not contain any architectural changes or does not introduce any new patterns and I think one review is sufficient'
• PR title is human readable and could be used in changelog

[Dschoordsch]

+1 Default to 9090 is obvious from the code, please remove the comment.
Useful would be why we've chosen 9090, for example 9090 is the standard prometheus port, if that's true

[Dschoordsch]

+1 (only start if ENABLE_METRICS is 'true') is obvious, please remove

[Dschoordsch]

-1 Why have a setting ENABLE_METRICS if we're collecting metrics unconditionally? Instead we should conditionally bind the function here. I suggest something along the lines of

// at the top of the file
const metricsEnabled = process.env.ENABLE_METRICS === 'true'

// here
open: !metricsEnabled ? handleOpen : (ws) => {
  trackOpen(ws)
  handleOpen(ws)
}

// metrics.ts
export const trackOpen = ((ws: WebSocket) => {
    incrementWebSocketConnections()
    trackWebSocketOpen(ws)
}

This way we don't need to know how to track things here and we don't spend any effort if tracking is disabled.