Create A2A monitoring guide

## Jobs to be done (optional)

Based on this Kapa issue https://app.kapa.ai/93b55f65-cffb-4d0b-9da7-c8915a838dce/conversations/63d36979-7212-4ac4-8424-64a639fe7c30

we need more complete guide's to supplement the reference docs here https://developer.konghq.com/plugins/ai-a2a-proxy/#logging-and-observability

## Definition of done

This could be a how-to similar to existing pages that take OTEL logging out to Prometheus or Grafana or it could be a full solution guide

## Size

- L (Large) Example: adding an API/Terraform how to, writing a reference page, writing a how to that involves a known third-party product (like Okta or Auth0)

Start with the A2A Proxy reference section on logging and observability, then review existing guides that send OTEL logging to Prometheus or Grafana. Decide whether the result is a how-to or a full solution guide, and ensure it provides a complete monitoring workflow for A2A.

Written by the indexing model from the issue text.