build(deps): update prometheus-client requirement

Updates the requirements on [prometheus-client](https://github.com/prometheus/client_python) to permit the latest version.
- [Release notes](https://github.com/prometheus/client_python/releases)
- [Commits](prometheus/client_python@v0.6.0...v0.21.0)

---
updated-dependencies:
- dependency-name: prometheus-client
  dependency-type: direct:production
...

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

optional-requirements.txt

@@ -9,7 +9,7 @@ python-olm>=3,<4
 unpaddedbase64>=1,<3
 
 #/metrics
-prometheus_client>=0.6,<0.20
+prometheus_client>=0.6,<0.22
 
 #/proxy
 pysocks