Ignore all metrics except set of metrics

[alvarocabanas]

As a user, I want to only collect a defined set of metric names and exclude everything else.

transformations:
  - description: allow
     ignore_metrics:
      - except:
        - apiserver_audit_requests_rejected_total

This behavior, even if it's contemplated in the tests, it's not working properly.

[workato-integration]

https://issues.newrelic.com/browse/NEWRELIC-3719

[paologallinaharbur]

Trying to reproducing it locally.
having:

  transformations:
    - description: allow
      ignore_metrics:
        - except:
            - go

seems to be working as expected

Moreover, there is a test covering that use case and the code seems without issues.

		exceptRulesLen += len(rule.Except)
		for _, prefix := range rule.Except {
			if strings.HasPrefix(name, prefix) {
				return false
			}
		}
[...]
		// Prefixes
		prefixesLen += len(rule.Prefixes)
		for _, prefix := range rule.Prefixes {
			if strings.HasPrefix(name, prefix) {
				return true
			}
		}

I am trying to verify if we reproduced it locally at some point

[paologallinaharbur]

The issue is related to lowDatamode and the way multiple rules are managed.

	if prefixesLen > 0 || metricTypesLen > 0 {
		return false
	}

This is considered at the end of all rules, therefore it is enough that one had a prefix to make useless a rule with only excludes

Having:

transformations:
- description: allow
  ignore_metrics:
  - except:
    - apiserver_audit_requests_rejected_total
- description: Low data mode defaults
  ignore_metrics:
  - prefixes:
    - kube_
    - container_
    - machine_
    - cadvisor_

Then becomes similar to:

- description: allow
  ignore_metrics:
  - except:
    - apiserver_audit_requests_rejected_total
  - prefixes:
    - kube_
    - container_
    - machine_
    - cadvisor_