Bump prometheus-client from 0.8.0 to 0.9.0

Bumps [prometheus-client](https://github.com/prometheus/client_python) from 0.8.0 to 0.9.0.
- [Release notes](https://github.com/prometheus/client_python/releases)
- [Commits](prometheus/client_python@v0.8.0...v0.9.0)

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

Author: dependabot-preview[bot]

poetry.lock

@@ -15,7 +15,7 @@ flake8 = "3.8.4"
 flask = "1.1.2"
 Flask-Cors = "3.0.9"
 Flask-Migrate = "2.5.3"
-prometheus_client = "0.8.0"
+prometheus_client = "0.9.0"
 psycopg2-binary = "2.8.6"
 py-healthcheck = "1.10.1"
 pytest = "6.2.1"