Provide a hook for metadata providers that need to generate their
metadata for each RPC. An example is authorization, where the provider
may need to refresh a token before it can provide the header.
Add stackdriver logging examplei to demonstrate calling a Google API.
Updated other examples to protobuf 0.6.0 (protoc plugin 0.7.8).
Updated SDK requirement to Dart 1.24.3, which adds support for creating a SecurityContext that trusts built-in roots, and support for ALPN on macOS.
Added an example showing how to send/receive custom metadata, and handle
call cancellation.
Implemented the remaining parts of metadata and cancel handling.
Addresses part of #8 and #12.
Jakob Andersen