Bump prometheus-client from 0.15.0 to 0.21.0

Bumps [prometheus-client](https://github.com/prometheus/client_python) from 0.15.0 to 0.21.0.
- [Release notes](https://github.com/prometheus/client_python/releases)
- [Commits](prometheus/client_python@v0.15.0...v0.21.0)

---
updated-dependencies:
- dependency-name: prometheus-client
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

requirements.txt

@@ -94,7 +94,7 @@ pickleshare==0.7.5
 Pillow~=9.4.0
 pluggy~=1.0.0
 portalocker~=2.7.0
-prometheus-client==0.15.0
+prometheus-client==0.21.0
 prompt-toolkit~=3.0.31
 protobuf~=4.21.8
 psutil~=5.9.3