Fix relabel key name

[sed-i]

Issue

Before this PR, during the config.yaml had the wrong key: metrics_relabel_configs (metrics in plural). The correct key is metric_relabel_configs (metric, in singular).

As a result, we were generating an invalid config that prometheus was dropping.

In addition, the yaml string wasn't parsed into a python object.

Solution

• Remove the stray s.
• yaml.safe_load the yaml configs.

In tandem with:

• https://github.com/canonical/prometheus-k8s-operator/pull/599

Testing

$ juju config sc metric_relabel_configs="$(cat <<EOF
- source_labels: ["__name__"]
  regex: "go_gc_.+"
  action: "drop"
EOF
)"

[sed-i]

Shouldn't we have a test that checks this configuration in Prometheus?

https://github.com/canonical/cos-lite-bundle/issues/107