Bump prometheus-client from 0.14.1 to 0.15.0

Bumps [prometheus-client](https://github.com/prometheus/client_python) from 0.14.1 to 0.15.0.
- [Release notes](https://github.com/prometheus/client_python/releases)
- [Commits](prometheus/client_python@v0.14.1...v0.15.0)

---
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]

requirements.txt

@@ -1,7 +1,7 @@
 raiden-common==0.1.8
 
 sentry-sdk[flask]==1.9.0
-prometheus_client==0.14.1
+prometheus_client==0.15.0
 networkx==2.8.7
 
 jaeger-client==4.8.0