fix(gax): implement getMessage() in FlowController exceptions

sdk-platform-java/gax-java/gax/src/main/java/com/google/api/gax/batching/FlowController.java

@@ -43,6 +43,10 @@ public class FlowController {
   /** Base exception that signals a flow control state. */
   public abstract static class FlowControlException extends Exception {
     private FlowControlException() {}
+
+    protected FlowControlException(String message) {
+      super(message);
+    }
   }
 
   /**
@@ -68,18 +72,16 @@ public static final class MaxOutstandingElementCountReachedException
     private final long currentMaxElementCount;
 
     public MaxOutstandingElementCountReachedException(long currentMaxElementCount) {
+      super(
+          String.format(
+              "The maximum number of batch elements: %d have been reached.",
+              currentMaxElementCount));
       this.currentMaxElementCount = currentMaxElementCount;
     }
 
     public long getCurrentMaxBatchElementCount() {
       return currentMaxElementCount;
     }
-
-    @Override
-    public String toString() {
-      return String.format(
-          "The maximum number of batch elements: %d have been reached.", currentMaxElementCount);
-    }
   }
 
   /**
@@ -91,18 +93,15 @@ public static final class MaxOutstandingRequestBytesReachedException
     private final long currentMaxBytes;
 
     public MaxOutstandingRequestBytesReachedException(long currentMaxBytes) {
+      super(
+          String.format(
+              "The maximum number of batch bytes: %d have been reached.", currentMaxBytes));
       this.currentMaxBytes = currentMaxBytes;
     }
 
     public long getCurrentMaxBatchBytes() {
       return currentMaxBytes;
     }
-
-    @Override
-    public String toString() {
-      return String.format(
-          "The maximum number of batch bytes: %d have been reached.", currentMaxBytes);
-    }
   }
 
   /**

sdk-platform-java/gax-java/gax/src/test/java/com/google/api/gax/batching/FlowControllerTest.java

@@ -774,4 +774,18 @@ private List<Future<?>> testConcurrentUpdates(
     executors.shutdown();
     return reserveReleaseFuture;
   }
+
+  @Test
+  void testExceptionGetMessage() {
+    FlowController.MaxOutstandingElementCountReachedException elementException =
+        new FlowController.MaxOutstandingElementCountReachedException(100);
+    assertThat(elementException.getMessage())
+        .isEqualTo("The maximum number of batch elements: 100 have been reached.");
+
+    FlowController.MaxOutstandingRequestBytesReachedException bytesException =
+        new FlowController.MaxOutstandingRequestBytesReachedException(500);
+    assertThat(bytesException.getMessage())
+        .isEqualTo("The maximum number of batch bytes: 500 have been reached.");
+  }
 }
+