build(deps): bump prometheus-client from 0.19.0 to 0.20.0

Bumps [prometheus-client](https://github.com/prometheus/client_python) from 0.19.0 to 0.20.0.
- [Release notes](https://github.com/prometheus/client_python/releases)
- [Commits](prometheus/client_python@v0.19.0...v0.20.0)

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

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

Author: dependabot[bot]

poetry.lock

@@ -20,7 +20,7 @@ openmldb-exporter = "openmldb_exporter.exporter:main"
 
 [tool.poetry.dependencies]
 python = "^3.8"
-prometheus-client = "^0.19.0"
+prometheus-client = ">=0.19,<0.21"
 openmldb = "^0.8.4"
 # uncomment below to use openmldb sdk built from source
 # set develop = true so changes in python will take effect immediately