feat: expose framework-neutral request metrics API

[gofurry]

• Add framework-neutral request metrics APIs:
  • RequestStarted()
  • RequestFinished(status, duration)
  • ObserveRequest(status, duration)
• Reuse the new lifecycle API in the existing net/http middleware path.
• Add Fiber, Gin, and Echo integration examples to English and Chinese README.
• Document the safe Fiber integration pattern to avoid creating monitor instances per request.