docs: add Prometheus metrics documentation

[Copilot]

PR #14 added Prometheus metrics support via the /metrics endpoint. This updates the README to document the new capability.

Changes

• Key Features: Added "Prometheus support" to the Metrics & Monitoring feature
• API Reference: New section documenting the /metrics endpoint with response format and exposed metrics (request counts, latencies, duration histograms, active requests)
• Usage Example: Added Example 6 showing curl access and Prometheus scrape configuration

scrape_configs:
  - job_name: 'fastpubsub'
    static_configs:
      - targets: ['localhost:8000']
    metrics_path: '/metrics'

Original prompt

Update the README.md with the recent changes of #14

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