Ability to provide additional variables to the metrictemplates custom resource

[njohnstone2]

Describe the feature

When specifying a custom metrics template there are a fixed set of variables (target, service, interval, etc). There are cases where the provided variables do provide enough information to filter metrics appropriately.

Proposed solution

Extend the canaries.flagger.app CRD to support additional metrics variables (optional) as key/value pairs to be exposed in the message template.

example: canary metric snippet (includes the proposed variables block)

spec:
  ...
  analysis:
    ...
    metrics:
    - name: kafka-lag
      variables:
        cluster: my-cluster
        consumer_group: my-consumer-group
      thresholdRange:
        max: 100
      interval: 1m

Any alternatives you've considered?

The alternative would be to update the metrics to ensure the variables available are exposed to apply filtering but this is not always feasible.