PodLogs: support static_labels stage

[rfratto]

The PodLogs CRD is missing support for Promtail's static_labels stage.

[tpaschalis]

I think that for now, a workaround would be to use a template stage to prepopulate the keys in the shared extracted map and then use a labels stage to set them on the log entry.

- template:
    source: myenv
    template: 'dev'
- labels:
    env: myenv