Environment variables in datasources.yaml

Hi, I’m trying to use datasources secrets as environment variables in k8s and I have problem with secret which is as certificate. If I paste it as plain text it works but not works with environment variable, others datasources works correctly.
Working config:

 datasources:
  datasources.yaml:
    apiVersion: 1
    datasources:
      - name: prometheus
        type: prometheus
        url: http://prometheus-operated:9090/
        access: proxy
        isDefault: true
      - name: "Google Cloud Monitoring"
        type: stackdriver
        access: proxy
        jsonData:
          tokenUri: "https://oauth2.googleapis.com/token"
          clientEmail: ${grafana_cloud_monitoring_email}
          authenticationType: "jwt"
          defaultProject: ${grafana_cloud_monitoring_project}
        secureJsonData:
          privateKey: |
            -----BEGIN PRIVATE KEY-----
            key
            -----END PRIVATE KEY-----

Not working config.

 datasources:
  datasources.yaml:
    apiVersion: 1
    datasources:
      - name: prometheus
        type: prometheus
        url: http://prometheus-operated:9090/
        access: proxy
        isDefault: true
      - name: "Google Cloud Monitoring"
        type: stackdriver
        access: proxy
        jsonData:
          tokenUri: "https://oauth2.googleapis.com/token"
          clientEmail: ${grafana_cloud_monitoring_email}
          authenticationType: "jwt"
          defaultProject: ${grafana_cloud_monitoring_project}
        secureJsonData:
          privateKey: ${grafana_cloud_monitoring_private_key}

How can I add privateKey as env variable when it is as certifcate? With one line secrets it works.