[feat][doc] add metrics for extensible load balancer

[Anonymitaet]

What change does this PR introduce?

This PR adds metrics for the extensible load balancer (topics in the red box below)

Doc development plan - Read before review

For the current doc of Broker Load Balancer, we want to tackle the following challenges:

• Create new documentation to cover all aspects of the feature (especially for the new extensible load balancer).
• Fill in any content gaps that may exist.
• Consolidate existing documentation into the new IA, as it is currently scattered and disorganized.
• Organize the documentation in a cohesive manner, ensuring it flows logically.

Consequently, we’ve created the following content development plans:

• Broker Load Balancing | Information Architecture
  This document presents a high-level architecture overview, including how we address content gaps and reconcile existing documentation.

• Broker Load Balancing | Doc Outline
  This document delves into the specifics of the content development plan, providing purpose and context for each section.

• doc

[Anonymitaet]

Previews go as expected:

[Anonymitaet]

@Demogorgon314 @heesung-sn @D-2-Ed @momo-jun @Huanli-Meng could you please help review this PR?

Thank you all!