logging: clear null trace_id (#2760)

pongad · web-flow · commit 8d646838b341 · 2018-01-18T12:39:03.000+11:00
To ThreadLocal, set(null) and remove() are functionally similar but remove() clears out the map-entry so it's more space-efficient. Fixes #2746.
diff --git a/google-cloud-logging/src/main/java/com/google/cloud/logging/TraceLoggingEnhancer.java b/google-cloud-logging/src/main/java/com/google/cloud/logging/TraceLoggingEnhancer.java
@@ -38,7 +38,11 @@ public TraceLoggingEnhancer(String prefix) {
    * @param id The traceID
    */
   public static void setCurrentTraceId(String id) {
-    traceId.set(id);
+    if (id == null) {
+      traceId.remove();
+    } else {
+      traceId.set(id);
+    }
   }
 
   /**
