code: func ExtractGRPC(md *metadata.MD) propagation.Extractor {
  	return func() (*model.SpanContext, error) {
  		var (
  			traceIDHeader      = GetGRPCHeader(md, TraceID)
  			spanIDHeader       = GetGRPCHeader(md, SpanID)
  			parentSpanIDHeader = GetGRPCHeader(md, ParentSpanID)
  			sampledHeader      = GetGRPCHeader(md, Sampled)
  			flagsHeader        = GetGRPCHeader(md, Flags)
  		)
  
  		return ParseHeaders(
  			traceIDHeader, spanIDHeader, parentSpanIDHeader, sampledHeader,
  			flagsHeader,
  		)
  	}
  }
docs: // ExtractGRPC will extract a span.Context from the gRPC Request metadata if
  // found in B3 header format.