Specifying // +protobuf.nullable=true on a Go type that is an alias of a
map or slice will generate a synthetic protobuf message with the type
name that will serialize to the wire in a way that allows the difference
between empty and nil to be recorded.
For instance:
// +protobuf.nullable=true
types OptionalMap map[string]string
will create the following message:
message OptionalMap {
map<string, string> Items = 1
}
and generate marshallers that use the presence of OptionalMap to
determine whether the map is nil (rather than Items, which protobuf
provides no way to delineate between empty and nil).
In order to allow a Go type to have a different internal representation,
provide the comment `// +protobuf.as=MESSAGENAME` which substitutes the
current message's members with the named message's members. Used by the
unversioned.Time type to have the same fields as unversioned.Timestamp
and to have an alternate serialization.
