Add a prerelease warning to EventListener (square#3580)

okhttp/src/main/java/okhttp3/EventListener.java

@@ -23,7 +23,14 @@
 import javax.annotation.Nullable;
 
 /**
- * EventListener for analytic events for an OkHttpClient instance.
+ * Listener for metrics events. Extend this class to monitor the quantity, size, and duration of
+ * your application's HTTP calls.
+ *
+ * <h3>Warning: This is a non-final API.</h3>
+ *
+ * <p><strong>As of OkHttp 3.9, this feature is an unstable preview: the API is subject to change,
+ * and the implementation is incomplete. We expect that OkHttp 3.10 or 3.11 will finalize this API.
+ * Until then, expect API and behavior changes when you update your OkHttp dependency.</strong>
  *
  * <p>All start/connect/acquire events will eventually receive a matching end/release event,
  * either successful (non-null parameters), or failed (non-null throwable).  The first common
@@ -275,6 +282,14 @@ public void callEnd(Call call) {
   public void callFailed(Call call, IOException ioe) {
   }
 
+  /**
+   * <h3>Warning: This is a non-final API.</h3>
+   *
+   * <p><strong>As of OkHttp 3.9, this feature is an unstable preview: the API is subject to change,
+   * and the implementation is incomplete. We expect that OkHttp 3.10 or 3.11 will finalize this
+   * API. Until then, expect API and behavior changes when you update your OkHttp
+   * dependency.</strong>
+   */
   public interface Factory {
     /**
      * Creates an instance of the {@link EventListener} for a particular {@link Call}. The returned