Deduplicate feature flag called events

Summary

Deduplicate $feature_flag_called events for repeated flag reads.

Why

sdk-specs marks feature-flag-called-tracker as applicable to both client and server SDKs. The tracker prevents noisy repeated $feature_flag_called events when application code reads the same flag/value repeatedly.

Current status

PostHog.FeatureFlags.Evaluations.enabled?/2 and get_flag/2 record access and fire $feature_flag_called on each call. No dedupe tracker was found.

Suggested scope

• Add an internal tracker keyed by distinct ID + flag key + evaluated value/result.
• Emit $feature_flag_called on the first read of a value.
• Suppress duplicate events for the same user/flag/value.
• Emit again when the value changes or the evaluation context is reset/reloaded.
• Keep memory bounded and concurrency-safe.
• Add tests for first read, duplicate read, and changed value.

References

• https://github.com/PostHog/sdk-specs/blob/main/openspec/specs/feature-flag-called-tracker/spec.md

Start with PostHog.FeatureFlags.Evaluations.enabled?/2 and get_flag/2, then read the sdk-specs feature-flag-called-tracker specification. Add tests covering the first read, duplicate read, changed value, context reset or reload, bounded memory, and concurrent access; done means repeated identical reads no longer emit duplicate events while changed evaluations emit again.