Issue #3242 - Allow Dumpable to control how self is represented

+ Do not rely on Object.toString() for representing self (aka:this)
  in the dump output.  Using Object.toString() can result in lots
  of duplicated information in the dump (and super long lines)

Signed-off-by: Joakim Erdfelt <joakim.erdfelt@gmail.com>

jetty-util/src/main/java/org/eclipse/jetty/util/component/Dumpable.java

@@ -70,6 +70,15 @@ public interface Dumpable
         return b.toString();
     }
 
+    /**
+     * The description of this/self found in the dump.
+     * Allows for alternative representation of Object other then .toString()
+     * where the long form output of toString() is represented in a cleaner way
+     * within the dump infrastructure.
+     *
+     * @return the representation of self
+     */
+    default String dumpSelf() { return toString(); }
 
     /**
      * Dump just an Object (but not it's contained items) to an Appendable.
@@ -90,6 +99,8 @@ public interface Dumpable
                 s = String.format("%s@%x[size=%d]",o.getClass().getComponentType(),o.hashCode(), Array.getLength(o));
             else if (o instanceof Map)
                 s = String.format("%s@%x{size=%d}",o.getClass().getName(),o.hashCode(),((Map<?,?>)o).size());
+            else if (o instanceof Dumpable)
+                s = ((Dumpable)o).dumpSelf().replace("\r\n","|").replace("\n","|");
             else
                 s = String.valueOf(o).replace("\r\n","|").replace("\n","|");