pubsub: Allow to set host in the client

[eddumelendez]

Hi, I want to write a test using testcontainers-go and as you can see here java client already allows to set the host and port, which is a random port provided by testcontainers. By setting the env var we are not able to run tests in parallel.

String hostport = emulator.getEmulatorEndpoint();
ManagedChannel channel = ManagedChannelBuilder.forTarget(hostport).usePlaintext().build();

[hongalex]

If I understand your concern correctly, you want to be able to specify multiple endpoints for your tests to run, while running within the same environment such that the env var option doesn't work for you, is that right?

I'm not super familiar with testcontainers, but the environment variable behavior (using PUBSUB_EMULATOR_HOST) is something we're interested in keeping. If you want to play around with custom options, you could use google.golang.org/api/option and option.WithEndpoint("localhost.com:1234") to specify a custom endpoint for your individual emulator instances when calling NewClient. You will also need to specify using insecure credentials

[eddumelendez]

@hongalex thank you! I follow your suggestions. You can see it here.