Topology labels are overridden in MetricsEndpointProvider

[lucabello]

Bug Description

When instantiating MetricsEndpointProvider and explicitly providing scrape jobs, specifying labels for topology under static_configs gets them replaced by the charm's topology.

For Mimir HA, we need the coordinator to set the worker's topology in the scrape job; however, setting the following:

jobs = [
  {
    "static_configs": [{
      "targets": [address],
      "labels": {
        "juju_application": worker_application,
        ...
      }
    }]
  }
]

the labels found in Prometheus contain the topology of the coordinator instead.

This shouldn't happen.

[lucabello]

We should solve this by not overriding labels in static_config if they're already specified in the scrape job, in the prometheus_scrape library.

As a workaround, currently this can be solved via relabel_configs.