Summary
Protocol Buffers version 3 reintroduced the optional keyword to track presence like proto2.
1
Fields without any label (known as "singular fields") will continue to omit presence information.
2
All fields are "optional" in that it is not an error if the sender fails to set them.
3
This feature allows you to track if a field has been set, even if the field is set to its default value.
4
According to
