fix: make OTEL evaluation event attribute types compliant to OTEL types

packages/shared/src/telemetry/attributes.ts

@@ -42,7 +42,6 @@ export const TelemetryAttribute = {
    *
    * - type: `undefined`
    * - requirement level: `conditionally required`
-   * - condition: variant is not defined on the evaluation details
    * - example: `#ff0000`; `1`; `true`
    */
   VALUE: 'feature_flag.result.value',

packages/shared/src/telemetry/evaluation-event.ts

@@ -1,8 +1,20 @@
-import { ErrorCode, StandardResolutionReasons, type EvaluationDetails, type FlagValue } from '../evaluation/evaluation';
-import type { HookContext } from '../hooks/hooks';
+import { ErrorCode, StandardResolutionReasons, type EvaluationDetails, type FlagValue } from '../evaluation';
+import type { HookContext } from '../hooks';
 import { TelemetryAttribute } from './attributes';
 import { TelemetryFlagMetadata } from './flag-metadata';
 
+/**
+ * Attribute types for OpenTelemetry.
+ * @see https://github.com/open-telemetry/opentelemetry-js/blob/fbbce6e1c0de86e4c504b5788d876fae4d3bc254/api/src/common/Attributes.ts#L35
+ */
+export declare type AttributeValue =
+  | string
+  | number
+  | boolean
+  | Array<null | undefined | string>
+  | Array<null | undefined | number>
+  | Array<null | undefined | boolean>;
+
 type EvaluationEvent = {
   /**
    * The name of the feature flag evaluation event.
@@ -13,7 +25,7 @@ type EvaluationEvent = {
    * @experimental The attributes are subject to change.
    * @see https://opentelemetry.io/docs/specs/semconv/feature-flags/feature-flags-logs/
    */
-  attributes: Record<string, string | number | boolean | FlagValue>;
+  attributes: Record<string, AttributeValue | undefined>;
 };
 
 const FLAG_EVALUATION_EVENT_NAME = 'feature_flag.evaluation';
@@ -36,12 +48,23 @@ export function createEvaluationEvent(
 
   if (evaluationDetails.variant) {
     attributes[TelemetryAttribute.VARIANT] = evaluationDetails.variant;
-  } else {
-    attributes[TelemetryAttribute.VALUE] = evaluationDetails.value;
+  }
+
+  if (evaluationDetails.value !== null) {
+    if (typeof evaluationDetails.value !== 'object') {
+      attributes[TelemetryAttribute.VALUE] = evaluationDetails.value;
+    } else {
+      try {
+        // Objects are not valid attribute values, so we convert them to a JSON string
+        attributes[TelemetryAttribute.VALUE] = JSON.stringify(evaluationDetails.value);
+      } catch {
+        // We ignore non serializable values
+      }
+    }
   }
 
   const contextId =
-    evaluationDetails.flagMetadata[TelemetryFlagMetadata.CONTEXT_ID] || hookContext.context.targetingKey;
+    evaluationDetails.flagMetadata[TelemetryFlagMetadata.CONTEXT_ID] ?? hookContext.context.targetingKey;
   if (contextId) {
     attributes[TelemetryAttribute.CONTEXT_ID] = contextId;
   }

packages/shared/test/telemetry.spec.ts

@@ -49,7 +49,7 @@ describe('evaluationEvent', () => {
     });
   });
 
-  it('should include variant when provided', () => {
+  it('should include variant and value when provided', () => {
     const details: EvaluationDetails<boolean> = {
       flagKey,
       value: true,
@@ -61,7 +61,24 @@ describe('evaluationEvent', () => {
     const result = createEvaluationEvent(mockHookContext, details);
 
     expect(result.attributes[TelemetryAttribute.VARIANT]).toBe('test-variant');
-    expect(result.attributes[TelemetryAttribute.VALUE]).toBeUndefined();
+    expect(result.attributes[TelemetryAttribute.VALUE]).toEqual(true);
+  });
+
+  it('should include object values as strings', () => {
+    const flagValue = { key: 'value' };
+
+    const details: EvaluationDetails<typeof flagValue> = {
+      flagKey,
+      value: flagValue,
+      variant: 'test-variant',
+      reason: StandardResolutionReasons.STATIC,
+      flagMetadata: {},
+    };
+
+    const result = createEvaluationEvent(mockHookContext, details);
+
+    expect(result.attributes[TelemetryAttribute.VARIANT]).toBe('test-variant');
+    expect(result.attributes[TelemetryAttribute.VALUE]).toEqual(JSON.stringify(flagValue));
   });
 
   it('should include flag metadata when provided', () => {