Draft: Add /metrics endpoint

What does this PR do?

Adds a Prometheus compatible /metrics endpoint.

As of right now, it only exposes metrics for the primary node process. This is not finished yet, but I'm opening it already to collect feedback / recommendations.

Still missing:

• collect metrics for all node processes (node:cluster)
• Add some form of access-control on the endpoint
• Queue metrics
  • Queue status (waiting/delayed/errors per queue)
  • Job timings (histogram, seperated by job)
  • Job state (like 4f987a37)
• Per-route request metrics (as implemented by fastify-metrics)
• Metrics endpoint in queue only nodes
• Federation stats

Contribution Guidelines By submitting this merge request, you agree to follow our Contribution Guidelines

• I agree to follow this project's Contribution Guidelines
• I have made sure to test this pull request