datastore: support conversion of protobuf ptypes directly

[ptone]

Consuming protobufs in an API (gRPC or otherwise) and then storing them in datastore is a really effective pattern, with a catch.

Protobuf supports some interesting ptypes for richer datatypes, such as timestamp, and duration.

These convert directly into similar verbose struct types in golang protobuf, but there exist helper utils to convert to native golang types

https://github.com/golang/protobuf/blob/master/ptypes/duration.go
https://github.com/golang/protobuf/blob/master/ptypes/timestamp.go

A proposal for similar support for structs was rejected
golang/protobuf#370

For these relatively limited number of ptypes, it would be nice if the conversion to golang native type, and then into datastore type was handled more easily, or perhaps an sample showing a protobuf PropertyLoadSaver implementation.

[jba]

I don't think the proto compiler will be changed to use native types. But writing a sample is definitely do-able.

[jba]

Moved to https://github.com/GoogleCloudPlatform/google-cloud-go/wiki/Feature-Backlog.

[timothyleung1]

I just store them directly in the datastore and it seems to work. It will create some weird fields such as XXX_cachsize etc tho..