feat(Dependencies): Update dependency prometheus-client to v0.21.0

| datasource | package           | from   | to     |
| ---------- | ----------------- | ------ | ------ |
| pypi       | prometheus-client | 0.19.0 | 0.21.0 |

requirements.txt

@@ -30,7 +30,7 @@ openstacksdk==2.1.0
 os-service-types==1.7.0
 pbr==6.0.0; python_version >= '2.6'
 pendulum==2.1.2; python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3, 3.4'
-prometheus-client==0.19.0
+prometheus-client==0.21.0
 pycparser==2.21; python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3'
 python-dateutil==2.9.0.post0; python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3'
 pytz-deprecation-shim==0.1.0.post0; python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3, 3.4, 3.5'