PreProcessors added medata hidden by default in Prometheus and Influx pushers

[tibosmn]

To the best of my understanding, only arguments specified in tags for Prometheus and Influx DB2 pushers are kept within the exposed PowerReports. However, when using PreProcessors such as K8sPreProcessor all the metadata added to PowerReports (such as namespace, container_id...) is not exported by default, which only keep sensor and target metadata.

I think this should be clarified in the documentation, along with a list of available tag labels

Thanks