Only insert labels that do not already exist

[sed-i]

Issue

Alert labels are being overwritten, when they already exist.
This is a problem for deployments with grafana-agent, such as:

graph LR
some-charm ---|logging| grafana-agent ---|logging| loki

Loading

Before this change, the alert labels of some-charm would be set to grafana-agent's topology.

Solution

Only insert labels that do not already exist. This is exactly what we're doing in cos-lib.

(In a different PR, we should refactor loki_push_api to use cos-lib.)

Addresses canonical/grafana-agent-k8s-operator#275.

Context

This PR is in tandem with:

• Fix label treatment of related charms grafana-agent-k8s-operator#277

Testing Instructions

Deploy this bundle:

bundle: kubernetes
applications:
  ga:
    # grafana-agent from https://github.com/canonical/grafana-agent-k8s-operator/pull/277
    # with the modified lib
    charm: ./grafana-agent-k8s_ubuntu-22.04-amd64.charm
    resources:
      agent-image: ubuntu/grafana-agent:0.35.2-22.04_stable
    scale: 1
    trust: true
  loki:
    charm: loki-k8s
    channel: edge
    series: focal
    scale: 1
    trust: true
  trfk:
    charm: traefik-k8s
    channel: edge
    series: focal
    scale: 1
    trust: true
relations:
- - trfk:logging
  - ga:logging-provider
- - ga:logging-consumer
  - loki:logging

and make sure expr labels and alert labels are correct:

juju show-unit loki/0 | yq '."loki/0"."relation-info" | .[] | ."application-data".alert_rules' | jq

Upgrade Notes

This change has no impact on Loki itself, but on related charms, so charmcraft fetch-lib etc.