add(cmd/ghz): expose --lb-strategy

[CAFxX]

Fixes #258 (at least the MVP)

[bojand]

Hello, thanks for the PR. Unfortunately load balancing in gRPC is a complicated topic. The reason #242 was API-only change is because even when using the included round robin LB strategy, additional setup might be (usually is) needed by the user. My understanding is that specifically a resolver would need to be registered via init side effect (See example). By default grpc-go uses a a passthrough resolver, so even setting the balancing strategy like this, depending on the configuration and hostname used, it would likely end up resulting in picking the first instance. The workaround is to use dns:///... scheme in passed hostname, or alternatively set the default resolver to "dns" (see docs).

So i think there is a little bit of additional work needed in this PR. in main() after we set up the config, where we create the logger, before Run() we should check if LB flag was set, and if it is round-robin we should set the resolver to dns using resolver.SetDefaultScheme("dns").

[CAFxX]

We can simply document that if the user wants to use client load balancing, she should specify the hostname with the dns:/// prefix. That's what we do internally.

Optionally we can add a warning if the hostname is missing the prefix.

[bojand]

Hello, thanks for the PR. This has been published in 0.91.0.

[mjgp2]

I don't think the dns:/// is documented fwiw