Use RUM header to allow distributed tracing

Regarding the ELK doc, we can use the header elastic-apm-traceparent
as trace_id for spans and transactions to allow distributed tracing
between our services.

Author: Kévin Liagre

src/Wrappers/Traits/EventTrait.php

@@ -69,7 +69,7 @@ private function initContext(EventSharedData $sharedData) : Context
     {
         // Generate Random UUIDs
         $this->id = UID::Generate(16);       //Uuid::uuid4()->toString();
-        $this->trace_id = UID::Generate(16); //Uuid::uuid4()->toString();
+        $this->trace_id = $this->trace_id = $_SERVER['HTTP_ELASTIC_APM_TRACEPARENT'] ? : UID::Generate(16); //Uuid::uuid4()->toString();
 
         // Set Shared Context Variable for further use
         $this->sharedData = $sharedData;
@@ -354,4 +354,4 @@ final protected function getCookieHeader(string $cookieHeader) : string
     }
 
 
-}
+}