log/field.go
func String(key, val string) Field {
	return Field{
		key:       key,
		fieldType: stringType,
		stringVal: val,
	}
}
func String(key, val string) Field {
	return Field{
		key:       key,
		fieldType: stringType,
		stringVal: val,
	}
}
// String adds a string-valued key:value pair to a Span.LogFields() record