code: func InjectGRPC(md *metadata.MD) propagation.Injector {
  	return func(sc model.SpanContext) error {
  		if (model.SpanContext{}) == sc {
  			return ErrEmptyContext
  		}
  
  		if sc.Debug {
  			setGRPCHeader(md, Flags, "1")
  		} else if sc.Sampled != nil {
  			// Debug is encoded as X-B3-Flags: 1. Since Debug implies Sampled,
  			// we don't send "X-B3-Sampled" if Debug is set.
  			if *sc.Sampled {
  				setGRPCHeader(md, Sampled, "1")
  			} else {
  				setGRPCHeader(md, Sampled, "0")
  			}
  		}
  
  		if !sc.TraceID.Empty() && sc.ID > 0 {
  			// set identifiers
  			setGRPCHeader(md, TraceID, sc.TraceID.String())
  			setGRPCHeader(md, SpanID, sc.ID.String())
  			if sc.ParentID != nil {
  				setGRPCHeader(md, ParentSpanID, sc.ParentID.String())
  			}
  		}
  
  		return nil
  	}
  }
docs: // InjectGRPC will inject a span.Context into gRPC metadata.