Add load balancer charts to Omnibus Grafana dashboard

We introduced NGINX VTS based metrics to Omnibus, but we don't have them in our Grafana dashboard yet.

We should add charts similar to what we do for GitLab.com for HA Proxy:

• Throughput
• Latency
• Error Rate

We could also include additional detail, if we can, on whether it's a backend or frontend request, etc.