Telemetry Publishing via Activity & Activity Event

[rossgrambo]

Why this PR?

412

We now have our own ActivitySource and add an ActivityEvent on it. The event is essentially the EvaluationEvent that was previously passed to the Publisher.

Why our own ActivitySource instead of using the parent Activity?

We could emit the ActivityEvent onto whatever parent Activity is running. Sadly this approach has a few drawbacks:

1. Listeners who want this event now have to listen to all activities.
2. The parent code may not have an activity
3. If a customer implements slow async logic (like reading targeting context from a db), it’s no longer grouped within FeatureManagement.

Visible Changes

1. Adding publishing uses a different method:

   builder.Services.AddFeatureManagement()
   .WithTargeting<HttpContextTargetingContextAccessor>()
   .AddTelemetryPublisher<ApplicationInsightsTelemetryPublisher>();

Becomes

builder.Services.AddFeatureManagement()
    .WithTargeting<HttpContextTargetingContextAccessor>()
    .AddAppInsightsTelemetryPublisher();

2. Public types ApplicationInsightsTelemetryPublisher & ITelemetryPublisher have been removed entirely
3. An Activity and Activity Event are available to listen to. Here's the schemas at a glance:

   Activity: {
   Name: "Microsoft.FeatureManagement",
   Version: "1.0.0"
   }

ActivityEvent: {
  Tags: {
    "TargetingId", evaluationEvent.TargetingContext?.UserId,
    "FeatureName", evaluationEvent.FeatureDefinition.Name,
    "Variant", evaluationEvent.Variant?.Name,
    "Enabled", evaluationEvent.Enabled,
    "VariantAssignmentReason", evaluationEvent.VariantAssignmentReason,
    "Version", ActivitySource.Version,
    ... // (Any items added to TelemetryMetadata)
  }
}

[jimmyca15]

Thanks for the thorough review on this @julealgon !