ipv6 addresses must be bracketed if using a port
ipv6 addresses must be bracketed if using a port

The host/port splitting code will not pull out a port if there are >=2 colons[1].
Unit test code demos this [2].  Note other documentation gets this right[3].  Example IP came from this issue[4] and the unit tests[2].
See also: relevant RFCs [5] [6]

[1]https://github.com/grpc/grpc/blob/618a3f561d4a93f263cca23abad086ed8f4d5e86/src/core/lib/gpr/host_port.cc#L81
[2]https://github.com/grpc/grpc/blob/618a3f561d4a93f263cca23abad086ed8f4d5e86/test/core/client_channel/resolvers/sockaddr_resolver_test.cc#L107
[3]https://chromium.googlesource.com/external/github.com/grpc/grpc/+/chromium-deps/2016-07-19/src/core/ext/client_config/README.md
[4]https://github.com/grpc/grpc/issues/6623
[5]http://www.iana.org/go/rfc5952#section-6
[6]http://www.iana.org/go/rfc3986#section-3.2.3
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