Bump prometheus-client from 0.17.1 to 0.20.0 in /tools/load-generator

Bumps [prometheus-client](https://github.com/prometheus/client_python) from 0.17.1 to 0.20.0.
- [Release notes](https://github.com/prometheus/client_python/releases)
- [Commits](prometheus/client_python@v0.17.1...v0.20.0)

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

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

tools/load-generator/requirements.txt

@@ -1,6 +1,6 @@
 PyYAML==6.0.1
 certifi==2023.7.22
 idna==3.4
-prometheus-client==0.17.1
+prometheus-client==0.20.0
 requests==2.31.0
 urllib3==1.26.18