code: public <T> T invoke(Callable<T> c) throws Exception {
          try {
              T result = c.call();
              succeed();
              return result;
          } catch (Exception e) {
              fail();
              throw e;
          }
      }
docs: Wrap the given service call with the {@link RequestCounter}
  to count the number of calls made.
  @param c the {@link Callable} to attempt
  
  @return whatever c would return on success
  
  @throws Exception if <code>c</code> throws one during
  execution