Describe the enhancement:

• add billing metricset in the Metricbeat azure module

Describe a specific use case for the enhancement or feature:

• make use of the azure sdk for go:
  https://github.com/Azure/azure-sdk-for-go/tree/master/services/consumption
  https://github.com/Azure/azure-sdk-for-go/tree/master/services/preview/billing/
• Billing APIs and Consumption APIs, for reference here https://docs.microsoft.com/en-us/rest/api/billing/, https://docs.microsoft.com/en-us/rest/api/consumption/#rest-operation-groups