Bump prometheus-client from 0.17.1 to 0.21.0

Bumps [prometheus-client](https://github.com/prometheus/client_python) from 0.17.1 to 0.21.0.
- [Release notes](https://github.com/prometheus/client_python/releases)
- [Commits](prometheus/client_python@v0.17.1...v0.21.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

pyproject.toml

@@ -12,7 +12,7 @@ gcn-kafka = "^0.3.3"
 python = "^3.9"
 ratelimit = "*"
 click = "^8.1.3"
-prometheus-client = "^0.17.0"
+prometheus-client = "^0.21.0"
 
 [tool.poetry.scripts]
 gcn-email = "gcn_email.cli:main"