code: def construct_xray_header(headers):
      
      header_str = headers.get(http.XRAY_HEADER) or headers.get(http.ALT_XRAY_HEADER)
      if header_str:
          return TraceHeader.from_header_str(header_str)
      else:
          return TraceHeader()
docs: Construct a ``TraceHeader`` object from dictionary headers
      of the incoming request. This method should always return
      a ``TraceHeader`` object regardless of tracing header's presence
      in the incoming request.