def extract(format, carrier)
      case format
      when OpenTracing::FORMAT_TEXT_MAP
        extract_from_text_map(carrier)
      when OpenTracing::FORMAT_BINARY
        warn 'Binary join format not yet implemented'
        nil
      when OpenTracing::FORMAT_RACK
        extract_from_rack(carrier)
      else
        warn 'Unknown join format'
        nil
      end
    end
def extract(format, carrier)
      case format
      when OpenTracing::FORMAT_TEXT_MAP
        extract_from_text_map(carrier)
      when OpenTracing::FORMAT_BINARY
        warn 'Binary join format not yet implemented'
        nil
      when OpenTracing::FORMAT_RACK
        extract_from_rack(carrier)
      else
        warn 'Unknown join format'
        nil
      end
    end
Extract a SpanContext from a carrier
 @param format [OpenTracing::FORMAT_TEXT_MAP, OpenTracing::FORMAT_BINARY, OpenTracing::FORMAT_RACK]
 @param carrier [Carrier] A carrier object of the type dictated by the specified `format`
 @return [SpanContext] the extracted SpanContext or nil if none could be found