expression {
  call_expr {
    function: "_[_]"
    args {
      select_expr {
        operand {
	  ident_expr {
	    name: "request"
	  }
        }
	field: "undefined"
      }
      const_expr {
        string_value: "foo"
      }
    }
  }
}
request_headers {
  headers{key: ":method" value : "GET"}
  headers{key: ":path" value : "/"}
  headers{key: ":scheme" value : "http"}
  headers{key: ":authority" value : "foo.com"}
  headers {}
  headers {}
  headers {}
  headers {}
  headers {}
}
response_headers {
  headers {
    key: ":status"
    value : "200"
  }
}
trailers {
  headers {}
}
stream_info {}