Support reading metadata when collecting EventCounter data #1287

Regarding the (new) Microsoft.ApplicationInsights.EventCounterCollector:

When I define a custom EventCounter I can add metadata to it:

methodDurationCounter = new EventCounter(nameof(methodDurationCounter), this);
methodDurationCounter.AddMetadata("Environment", "Production");
methodDurationCounter.AddMetadata("Customer", "Ibis");

This metadata is added to the payload. I would be very happy when this data is included as custom properties of the MetricTelemetry that is send. See the source.

I would be more than willing to provide a PR for this.