limit/rate: add rate() and is_limited() inspector methods on RateLimit service

[Aditya-9-6]

Summary

This PR adds public inspector methods rate(&self) and is_limited(&self) to RateLimit<T> in tower/src/limit/rate/service.rs.

Rationale & Value

Currently, callers using tower::limit::RateLimit cannot inspect the rate configuration or check if the rate limiter is in State::Limited without polling the service.

Adding rate(&self) and is_limited(&self):

• Exposes zero-cost state inspection for metrics, logging, and health checks.
• Avoids forcing unnecessary poll_ready state transitions when inspecting rate limiter status.