code: public synchronized void addChildContext(String childContextName, final MetricContext childContext)
        throws NameConflictException, ExecutionException {
      if (this.children.get(childContextName, new Callable<MetricContext>() {
        @Override
        public MetricContext call() throws Exception {
          return childContext;
        }
      }) != childContext) {
        throw new NameConflictException("A child context with that name already exists.");
      }
    }
docs: Add a {@link MetricContext} as a child of this {@link MetricContext} if it is not currently a child.
  
  @param childContextName the name of the child {@link MetricContext}
  @param childContext the child {@link MetricContext} to add