code: public ClientFactoryBuilder http2InitialConnectionWindowSize(int http2InitialConnectionWindowSize) {
          checkArgument(http2InitialConnectionWindowSize >= DEFAULT_WINDOW_SIZE,
                        "http2InitialConnectionWindowSize: %s (expected: >= %s and <= %s)",
                        http2InitialConnectionWindowSize, DEFAULT_WINDOW_SIZE, MAX_INITIAL_WINDOW_SIZE);
          this.http2InitialConnectionWindowSize = http2InitialConnectionWindowSize;
          return this;
      }
docs: Sets the <a href="https://tools.ietf.org/html/rfc7540#section-6.9.2">initial connection flow-control
  window size</a>. The HTTP/2 connection is first established with
  {@value Http2CodecUtil#DEFAULT_WINDOW_SIZE} bytes of connection flow-control window size,
  and it is changed if and only if {@code http2InitialConnectionWindowSize} is set.
  Note that this setting affects the connection-level window size, not the window size of streams.
  
  @see #http2InitialStreamWindowSize(int)