Bump prometheus-client from 0.20.0 to 0.21.0

Bumps [prometheus-client](https://github.com/prometheus/client_python) from 0.20.0 to 0.21.0.
- [Release notes](https://github.com/prometheus/client_python/releases)
- [Commits](prometheus/client_python@v0.20.0...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

@@ -3,7 +3,7 @@ chewie==0.0.25
 eventlet>=0.34.3; python_version >= "3.12"
 networkx>=1.9
 pbr>=1.9
-prometheus_client==0.20.0
+prometheus_client==0.21.0
 ruamel.yaml==0.18.6
 os_ken==2.9.0
 beka==0.4.2