admin: implement admin services

[menghanl]

Admin service contains channelz and CSDS.
CSDS is registered by the xds package, so that the service is registered (and dependency is imported) only when xds is imported.

This PR also moves some packages from xds/internal to internal/xds, for testing purposes

• internal/xds/env
• testing bootstrap generation

[dfawley]

I think it might be nicer to pass t to RunRegisterTests so it will print the proper line where the failure happened.

You can also do a t.Run then.

[menghanl]

Done.

How to do t.Run?

[dfawley]

t.Run("channelz", func(t *testing.T) {
	if err := RunChannelz(conn); status.Code(err) != ec.ChannelzCode {
		t.Fatalf("%s test failed with error %v, want code %v", "channelz", err, ec.ChannelzCode)
	}
})
t.Run("csds", func(t *testing.T) {
	if err := RunCSDS(conn); status.Code(err) != ec.CSDSCode {
		t.Fatalf("%s test failed with error %v, want code %v", "CSDS", err, ec.CSDSCode)
	}
})

[menghanl]

Done. Maybe we don't need "%s test failed" any more? But I kept them.

[dfawley]

Maybe remove the name? Just make it a slice?

[menghanl]

I wanted to avoid duplicates, because they panic:

grpc-go/server.go

Lines 596 to 598 in 967933b

	if _, ok := s.services[sd.ServiceName]; ok {
	logger.Fatalf("grpc: Server.RegisterService found duplicate service registration for %q", sd.ServiceName)
	}

[dfawley]

But why would we register the same service multiple times?

Also there's no guarantee that the name used when registering with admin is the same as the name used when registering the service with the server.

[menghanl]

Maybe this should take a list of grpc.ServiceDesc, then we can dedup based on the service name.

[menghanl]

That doesn't work yet, because CSDS's service desc isn't exported.
Removed name. This is internal anyway, should be safe.