Using ResponseInterceptor : add request execution elapsedTime to the InvocationContext

I'm implementing the ResponseInterceptor interface to provide informations about each HTTP request to my users.
I wanted to display the request execution elapsedTime, but this data is not present in the InvocationContext.

in ResponseHandler, this could be as simple as

public Object handleResponse(
      String configKey, Response response, Type returnType, long elapsedTime) throws Exception {
    try {
      response = logAndRebufferResponseIfNeeded(configKey, response, elapsedTime);
      return executionChain.next(
          new InvocationContext(
              configKey,
              decoder,
              errorDecoder,
              dismiss404,
              closeAfterDecode,
              decodeVoid,
              response,
              returnType,
              elapsedTime));

Start by reading ResponseHandler.handleResponse and the InvocationContext and ResponseInterceptor APIs named in the issue. Trace how the elapsedTime argument is currently used, then expose it through the invocation context so response interceptors can access it. Done means an interceptor can read the HTTP request execution elapsed time from InvocationContext.