Cluster Agent not honoring Site parameter when MetricsProvider is queried

[mantoine96]

Describe what happened:

I'm using Datadog Cluster Agent with metricsProviderEnabled: true and Site: datadoghq.eu

After configuring an HPA, I started seeing these errors:

2020-05-28 11:17:23 UTC | CLUSTER | ERROR | (pkg/util/kubernetes/autoscalers/datadogexternal.go:84 in queryDatadogExternal) | Error while executing metric query avg:eng_kyc_service.process.files.open{cluster_name:dev-eu-west-1-test,env:dev}.rollup(30): API error 403 Forbidden: {"errors": ["Invalid API key or Application key"]}
2020-05-28 11:17:23 UTC | CLUSTER | ERROR | (pkg/util/kubernetes/autoscalers/processor.go:80 in UpdateExternalMetrics) | Error getting metrics from Datadog: [Error while executing metric query avg:eng_kyc_service.process.files.open{cluster_name:dev-eu-west-1-test,env:dev}.rollup(30): API error 403 Forbidden: {"errors": ["Invalid API key or Application key"]}, strconv.Atoi: parsing "": invalid syntax]

I verified my credentials and they are valid, as long as you point to datadoghq.eu. Setting DATADOG_HOST=https://app.datadoghq.eu solved this.

Describe what you expected:

I would expect one of two things:

• DATADOG_HOST is populated from DD_SITE and DD_DD_URL when creating the Cluster Agent deployment
• Datadog Cluster Agent's client library leverages DD_SITE and DD_DD_URL to create the client.

Steps to reproduce the issue:

• Create a Datadog Cluster agent deployment with datadoghq.eu credentials and `metricsProviderEnablk
• Create an HPA using an external metric

Additional environment details (Operating System, Cloud provider, etc):