log/field.go
func Int64(key string, val int64) Field {
	return Field{
		key:        key,
		fieldType:  int64Type,
		numericVal: val,
	}
}
func Int64(key string, val int64) Field {
	return Field{
		key:        key,
		fieldType:  int64Type,
		numericVal: val,
	}
}
// Int64 adds an int64-valued key:value pair to a Span.LogFields() record