Don't run ExP related logic if use_edge_uniques is not enabled

Ottomatarequested to merge
T396359_exp_malformedheader_error into master

The Experiment Platform logic in makeSetWikimediaDefaults was previously always run if the X-Experiment-Enrollments header was set. This caused error events (not ExP events) to fail to be produced in this case, since they are produced in the same HTTP request as the incoming ExP event.

This change refactors the logic to only check and the hoist the X-Experiement-Enrollments if the producers.eventgate.use_edge_uniques stream config setting is set.

Bug: T396359

Edited by Ottomata

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