Add channelz support and the admin interface

[murgatroid99]

Add channelz functionality to grpc-js (see gRFC A14), along with the admin interface (see gRFC A38).

There are three main parts of this change:

• The channelz bookkeeping logic, which is spread among the first part of channelz.ts, channel.ts, subchannel.ts, and server.ts, with small parts in call-stream.ts and server-call.ts to count messages.
• The channelz service handler code, which all lives in the second part of channelz.ts.
• The admin interface, which is simply a registry of service definitions and corresponding service handlers to be added to a server as a group using the addAdminServicesToServer function. The registration function takes getters instead of plain objects to support retrieving those things lazily, which allows us to load the channelz service lazily so that we do not need to load protobufjs in applications that do not use channelz.

This change adds the following public APIs:

• getChannelzServiceDefinition(): ServiceDefinition
• getChannelzHandlers(): ServiceHandler
• addAdminServicesToServer(server: Server): void
• Channel#getChannelzRef(): ChannelRef
• Server#getChannelzRef(): ServerRef

The intended use is for a user to do one of the following to serve the channelz service:

const server = new grpc.Server();
server.addService(grpc.getChannelzServiceDefinition(), grpc.getChannelzHandlers());

OR

const server = new grpc.Server();
grpc.addAdminServicesToServer(server);

In addition, I modified the channel, subchannel, and server trace logs to include the channelz ID to make them easier to cross-reference.

The new channelz_manual_test.js file just starts a server and a client that continuously makes requests to that server, and starts another server serving the channelz service, for a separate channelz client to examine.

[nicolasnoble]

It'd still be great at some point if we had a common repository for those...

[murgatroid99]

Well, there is actually https://github.com/grpc/grpc-proto