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]>

setup.py

@@ -54,7 +54,7 @@ def read(path: str) -> str:
         "kopf==1.35.6",
         "kubernetes-asyncio==31.1.0",
         "PyYAML<7.0",
-        "prometheus_client==0.20.0",
+        "prometheus_client==0.21.0",
         "aiohttp==3.10.8",
         "wrapt==1.16.0",
     ],