atlas: fix duplicate flush on shutdown (#1071)

If it was in the process of sending data when the registry
is shutdown, then it could result in the same time interval
getting sent multiple times. Synchronizing the methods is
a simple solution for now as only a single thread should
be performing a send at a given time.

spectator-reg-atlas/src/main/java/com/netflix/spectator/atlas/AtlasRegistry.java

@@ -244,7 +244,7 @@ private Timer publishTaskTimer(String id) {
     return debugRegistry.timer(PUBLISH_TASK_TIMER, "id", id);
   }
 
-  void sendToAtlas() {
+  synchronized void sendToAtlas() {
     publishTaskTimer("sendToAtlas").record(() -> {
       if (config.enabled()) {
         long t = lastCompletedTimestamp(stepMillis);
@@ -272,7 +272,7 @@ void sendToAtlas() {
     });
   }
 
-  void sendToLWC() {
+  synchronized void sendToLWC() {
     publishTaskTimer("sendToLWC").record(() -> {
       long t = lastCompletedTimestamp(lwcStepMillis);
       //if (config.enabled() || config.lwcEnabled()) {