Describe the enhancement: Adding an appinsights metricset in the Metricbeat azure module and appinsights fileset in the Filbeat azure module.

Describe a specific use case for the enhancement or feature:

• use the azure sdk for go: https://github.com/Azure/azure-sdk-for-go/tree/master/services/preview/appinsights/v1/insights which matches the appinsights rest
  apis https://dev.applicationinsights.io/quickstart
• for the appinsights metricset we should use the GET /metrics api which contains the app's telemetry data aggregated during ingestion into the Application Insights, some similarities to the monitor metricset will be there mostly related to filtering.
• for the other 2 apis in appinsights: GET events API and query API return event data, either individual events or aggregated with a range of options/filters, maybe we should consider creating an appisinsights fileset as well for the events