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.
First stage of separating Connection from Channel. A Channel manages
multiple Connections, and chooses which Connection to send an RPC on.
In this change, the Channel still creates a Connection for each RPC.
Managing the Connection life-cycle comes in a later change.
Implements the basic server interoperability tests for gRPC compliance.
Compressed tests, TLS, and authentication are not implemented yet, but
basic test cases pass against the C++ gRPC interoperability client.
Kevin Moore