Add remote_ip to the context from Labkit::Middleware::Rack

For Rails, we're currently manually adding the remote ip to the context from several places in the application:

• Gitlab::GrapeLogging::Loggers::ContextLogger
• ApplicationController

This means that in some places, for example, middlewares (gitlab-org/gitlab!169124 (comment 2194904501)), we don't have this information in the context yet when we emit logs. We'd also have to re-implement it in CustomersDot.

Ideally, we'd already add this in Labkit::Middleware::Rack, for this, we'd also need the ActionDispatch::RemoteIp middleware, similar to how we also need ActionDispatch::RequestId for the correlation id.

I think we could move this into Labkit entirely, so the client applications no longer need to do this.