log/field.go
func Uint64(key string, val uint64) Field {
	return Field{
		key:        key,
		fieldType:  uint64Type,
		numericVal: int64(val),
	}
}
func Uint64(key string, val uint64) Field {
	return Field{
		key:        key,
		fieldType:  uint64Type,
		numericVal: int64(val),
	}
}
// Uint64 adds a uint64-valued key:value pair to a Span.LogFields() record