issue411

type Span struct {
	TraceID              TraceID  `protobuf:"bytes,1,opt,name=trace_id,json=traceId,proto3,customtype=TraceID" json:"trace_id"`
	SpanID               SpanID   `protobuf:"bytes,2,opt,name=span_id,json=spanId,proto3,customtype=SpanID" json:"span_id"`
	XXX_NoUnkeyedLiteral struct{} `json:"-"`
	XXX_unrecognized     []byte   `json:"-"`
	XXX_sizecache        int32    `json:"-"`
}