chore(deps): bump prometheus-client in /chainguard/environments/dev

Bumps [prometheus-client](https://github.com/prometheus/client_python) from 0.16.0 to 0.21.0.
- [Release notes](https://github.com/prometheus/client_python/releases)
- [Commits](prometheus/client_python@v0.16.0...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]>

chainguard/environments/dev/requirements.txt

@@ -1,4 +1,4 @@
 Flask==3.0.3
 redis==5.1.0b7
-prometheus-client==0.16.0
+prometheus-client==0.21.0
 Werkzeug==3.0.6