Added new load balancer dashboard for pulsar 31

[mukesh154]

Motivation

1. Some load balancer metrics have been changed from pulsar 2.10 to pulsar 3.1.

   • pulsar_lb_unload_broker_count -> pulsar_lb_unload_broker_total
   • pulsar_lb_unload_bundle_count -> pulsar_lb_unload_bundle_total
   • pulsar_lb_bundles_split_count -> pulsar_lb_bundles_split_total
     Ref: click here

2. The Pulsar Monitor & Pulsar Heartbeat dashboards were using the same uid due to which Pulsar Monitor dashboard was not available to grafana.

[pgier]

This is out of date, so it's been replaced by #210