docs: fix Prometheus metrics types in documentation

[janhalama]

Description

Fix Prometheus metrics types in documentation according to the reality and add link to Prometheus metrics type documentation.

This is a

• [ ] Bug Fix
• [ ] Feature
• [x] Documentation
• [ ] Other

Checklists

Commit style

• [x] Changes are on a branch with a descriptive name eg. fix/missing-queue, docs/setup-guide

• [x] Commits start with one of feat: fix: docs: chore: or similar

• [x] No excessive commits, eg: there should be no fix: commits for bugs that existed only on the PR branch (see guide-to-interactive-rebasing)

Protected files

The following files should not change unless they are directly a part of your change.

• [ ] yarn.lock (unless package.json is also modified, then only the new/updated package should be changed here)

• [ ] package.json (renovate bot should handle all routine updates)

• [ ] package-lock.json (Should not exist as this project uses yarn)

• [ ] tsconfig.json (only make it stricter, making it more lenient requires more discussion)

• [ ] tslint.json (only make it stricter, making it more lenient requires more discussion)