Place filename into structured_metadata

[Abuelodelanada]

Issue

This PR takes advantage of structured_metadata in Loki. See: canonical/loki-k8s-operator#445

Solution

This PR removes filename from labels and place it into structured_metadata in order to avoid high cardinality.

Testing Instructions

Run integration tests

[Abuelodelanada]

Since we have learn something that was not documented in Grafana docs, which is that grafana-agent is not able to forward structured_metadata coming from promtail (or another grafana-agent) to loki

promtail --> grafana-agent --> loki

We are closing this PR because the pipeline is not reliable. An administrator may wrongly think that because promtail sends structured_metadata it will reach loki when it is not always true.