add support for other projects for external metrics

[astropuffin]

Addresses #205

I tried to keep my changes as minimal as possible as I don't really have the bandwidth or expertise to do a large refactor. This has been tested in my environment to maintain existing functionality, as well as access to metrics in other projects. Adding scope was necessary to allow a service account to work, rather than GCE creds.

I would be happy to contribute documentation if this PR's code is deemed mergable.

Example spec:

apiVersion: autoscaling/v2beta1
kind: HorizontalPodAutoscaler
metadata:
  name: dummy
  namespace: default
spec:
  scaleTargetRef:
    apiVersion: apps/v1beta1
    kind: Deployment
    name: dummy
  minReplicas: 1
  maxReplicas: 5
  metrics:
  - external:
      metricName: pubsub.googleapis.com|subscription|num_undelivered_messages
      metricSelector:
        matchLabels:
          resource.labels.subscription_id: super-awesome-sub
          resource.labels.project_id: other-project
      targetAverageValue: "2"
    type: External

Note: Access to other projects requires running the stackdriver adapter with a service account (recommended), or adding permissions to the default GCE creds (not advised).

[kawych]

Please make sure that req.Operator() is selection.Equals (or selection.DoubleEquals), otherwise an error should be returned.

[kawych]

Thanks for contribution! I have a small comment, but LGTM otherwise. Please update the documentation to reflect this.

[astropuffin]

@kawych Added documentation. Let me know if it needs anything else, whether stylistic or content. I followed the existing examples docs.

[kawych]

Thanks. Can you file a separate PR for the documentation? Sorry for not being clear with that, I want to submit it only after we do a next release of the adapter.

[astropuffin]

@kawych done. See #213

[kawych]

LGTM

[bjorndv]

I've noticed that the last release is from 18 June (custom-metrics-stackdriver-adapter v0.8.0). Are there any plans to roll out a new release soon? I could really use this feature.

[kawych]

I've started a release of the new version. Should be available in several days.

[bjorndv]

Any update on the release? Thanks!