def extract(format, carrier)
      case format
      when OpenTracing::FORMAT_TEXT_MAP, OpenTracing::FORMAT_BINARY, OpenTracing::FORMAT_RACK
        return SpanContext::NOOP_INSTANCE
      else
        warn 'Unknown extract format'
        nil
      end
    end
def extract(format, carrier)
      case format
      when OpenTracing::FORMAT_TEXT_MAP, OpenTracing::FORMAT_BINARY, OpenTracing::FORMAT_RACK
        return SpanContext::NOOP_INSTANCE
      else
        warn 'Unknown extract format'
        nil
      end
    end
Extract a SpanContext in the given format from the given 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, nil] the extracted SpanContext or nil if none could be found