Bump prometheus_exporter version

[dominikl]

Bump omero_prometheus_exporter version to latest 0.4.0.

Otherwise you run into this issue on Rocky9:

TASK [ome.omero_prometheus_exporter : omero prometheus exporter | install python3] ****************************************************************************************
fatal: [68090143-9473-4169-8a4e-86888b22a4f3]: FAILED! => {"changed": false, "failures": ["No package python36 available."], "msg": "Failed to install some of the specified packages", "rc": 1, "results": []}

[sbesson]

Thanks @dominikl cherry-picked in #413 alongside the other Rocky 9 commits. Should we close this?

[dominikl]

👍