C2C WSGI Utils 6.0 released

[sbrunner]

The major change is that we replace StatsD and custom Prometheus stats with stats based on the official Prometheus Python Client.

For that, a service on the C2CWSGIUTILS_PROMETHEUS_PORT is opened, and some configuration should be added in the Gunicorn configuration file.

For more information, see the documentation.

Concerned pull request: #1852

This reorganization probably needs an adaptation on your side: Reorganize the dependency #1945

Nose addition that can be useful for your tests: Add the possibility to check an URL directly #1946

Nice to have: Automatically use the dark mode #1931