Track evaluation analytics custom events

[Zaimwa9]

To be scoped

Alpha state

The evaluation analytics pipeline is functional in alpha (@flagsmith/flagsmith@internalv11.0.0). Currently we track flag evaluations triggered on every hasFeature() / getValue() call emits an event with data available at evaluation time

We need to define the scope and the behavior of tracking custom events as it's been discussed.

Right now the event schema looks like:

interface IPipelineEvent {
      event_id: string; => flag key
      event_type: 'flag_evaluation' | 'custom_event'; => will hold the custom event
      evaluated_at: number;
      identity_identifier: string | null;
      enabled?: boolean | null;
      value: IFlagsmithValue;
      traits?: { [key: string]: null | TraitEvaluationContext } | null;
      metadata?: Record<string, any> | null;
  }

Open questions

• Do we want to provide a generic trackEvent method that allows end user to track any event he wants
• How do we tie flag evaluations with a custom event to stay in feature flagging territory?

[matthewelwell]

The expectation here is to implement basic custom event tracking similar to what we had implemented here. The event data should, however, reach the event streaming service where we are currently sending flag analytics data as part of this parent epic.

We will need to flesh this issue out with more details on that, but we will estimate based on this requirement.