Apply suggestions from code review

Co-authored-by: Karsten Schnitter <k.schnitter@sap.com>
Signed-off-by: David Venable <dlv@amazon.com>

data-prepper-plugins/user-agent-processor/src/main/java/org/opensearch/dataprepper/plugins/processor/useragent/UserAgentProcessor.java

@@ -43,8 +43,8 @@ public UserAgentProcessor(
         super(pluginMetrics);
         this.config = config;
         this.userAgentParser = new CaffeineCachingParser(config.getCacheSize());
-        sourceKey = config.getSource();
-        targetKey = eventKeyFactory.createEventKey(config.getTarget(), EventKeyFactory.EventAction.PUT);
+        this.sourceKey = config.getSource();
+        this.targetKey = eventKeyFactory.createEventKey(config.getTarget(), EventKeyFactory.EventAction.PUT);
     }
 
     @Override