Fill the stacktrace of ResponseCompleteException when `verboseRespo…

…nses = true`

Motivation:

`ResponseCompleteException` is considered a safe exception to clean up
request resources. However, it would be a good idea to leave a
stacktrace in case there is a bug in the implementation or the user
wants to know how the error occurred.

Modifications:

- Create a new instance if `verboseResponse = true` and
  `ResponseCompletionException` is sampled by `verboseExceptionSampler`

Result:

You can now sees the stacktrace of `ResponseCompleteException` when
`Flags.verboseResponse()` is true.

core/src/main/java/com/linecorp/armeria/common/ResponseCompleteException.java

@@ -26,16 +26,23 @@ public final class ResponseCompleteException extends CancellationException {
 
     private static final long serialVersionUID = 6090278381004263949L;
 
-    private static final ResponseCompleteException INSTANCE = new ResponseCompleteException();
+    private static final ResponseCompleteException INSTANCE = new ResponseCompleteException(false);
 
     /**
      * Returns the singleton {@link ResponseCompleteException}.
      */
     public static ResponseCompleteException get() {
-        return INSTANCE;
+        if (Flags.verboseResponses() &&
+            Flags.verboseExceptionSampler().isSampled(ResponseCompleteException.class)) {
+            return new ResponseCompleteException();
+        } else {
+            return INSTANCE;
+        }
     }
 
-    private ResponseCompleteException() {
+    private ResponseCompleteException() {}
+
+    private ResponseCompleteException(@SuppressWarnings("unused") boolean dummy) {
         super(null, null, false, false);
     }
 }