instrumentation: Avoid user.sessionId() if possible, since it sets a cookie

Bug: T289780
Change-Id: I8d80f4e02c6b59087c88d42be671fa81b4d37ba8

src/instrumentation/statsv.js

@@ -17,6 +17,10 @@
  */
 export function isEnabled( user, config, experiments ) {
 	const bucketingRate = config.get( 'wgPopupsStatsvSamplingRate', 0 );
+	if ( bucketingRate === 0 || bucketingRate === 1 ) {
+		// Avoid calling user.sessionId() if possible, since it sets a cookie
+		return !!bucketingRate;
+	}
 
 	return experiments.weightedBoolean(
 		'ext.Popups.statsv',

tests/node-qunit/instrumentation/statsv.test.js

@@ -34,11 +34,15 @@ QUnit.test( '#isEnabled', function ( assert ) {
 
 	config.delete( 'wgPopupsStatsvSamplingRate' );
 
-	isEnabled( user, config, experiments );
-
-	assert.deepEqual(
-		weightedBooleanStub.getCall( 1 ).args[ 1 ],
-		0,
-		'The bucketing rate should be 0 by default.'
+	const defaultResult = isEnabled( user, config, experiments );
+	assert.strictEqual(
+		defaultResult,
+		false,
+		'The bucketing is disabled by default.'
+	);
+	assert.strictEqual(
+		weightedBooleanStub.callCount,
+		1,
+		'The experiments object is not called when bucketing rate is the default 0.'
 	);
 } );