[Feature Request] Add metadata.messageType to protobuf payloads

[lorensr]

To match these PRs in Java SDK (with explanation) and Go SDK.

Additional context

I'm not sure how to get the message type / full name (my.package.MyMessage) from the message instance. Google's PHP Protobuf library doesn't seem to be documented, but perhaps message.desc.getFullName()? $this->desc is private:

https://github.com/protocolbuffers/protobuf/blob/master/php/src/Google/Protobuf/Internal/Message.php#L91-L94

but perhaps it's still accessible? https://stackoverflow.com/a/2448499/627729