warn if SpectatorContext registry is overwritten (#458)

Adds a warning level log message if the registry used with
the SpectatorContext is overwritten. This should help with
debugging cases where Servo metrics are not being reported
properly because of initialization bugs in the application.

servo-core/src/main/java/com/netflix/servo/SpectatorContext.java

@@ -69,11 +69,29 @@ private SpectatorContext() {
 
   private static volatile Registry registry = new NoopRegistry();
 
+  // Used to keep track of where the registry was being set. This can be useful to help
+  // debug issues if missing metrics due to it being set from multiple places.
+  private static volatile Exception initStacktrace = null;
+
   /**
    * Set the registry to use. By default it will use the NoopRegistry.
    */
   public static void setRegistry(Registry registry) {
     SpectatorContext.registry = registry;
+    if (registry instanceof NoopRegistry) {
+      initStacktrace = null;
+    } else {
+      Exception cause = initStacktrace;
+      Exception e = new IllegalStateException(
+          "called SpectatorContext.setRegistry(" + registry.getClass().getName() + ")",
+          cause);
+      e.fillInStackTrace();
+      initStacktrace = e;
+      if (cause != null) {
+        LOGGER.warn("Registry used with Servo's SpectatorContext has been overwritten. This could "
+            + "result in missing metrics.", e);
+      }
+    }
   }
 
   /**