Adding prometheus metrics to the interceptor

[arschles]

Adds prometheus instrumentation and a fetch endpoint for those instrumentation

Checklist

• [x] Commits are signed with Developer Certificate of Origin (DCO)
• [x] Any necessary documentation is added, such as:
  • README.md
  • The docs/ directory
  • The docs repo

Fixes #321

[tomkerkhove]

@arschles Should we abandon this PR or not?

[JorTurFer]

This is already implemented