limits: add peek and proportional_rate methods to Rate

[Aditya-9-6]

Summary

Adds non-mutating rate inspection methods (peek and proportional_rate) to pingora_limits::rate::Rate.

Motivation & Problem

When building WAF proxies, traffic inspectors, and multi-stage rate-limiting middleware using pingora-limits, callers frequently need to inspect / dry-run current event counts or sliding window rates for IP reputation scoring and logging without incrementing the rate counter via observe().

Previously, callers had no way to non-destructively peek into the current interval event count, and using PROPORTIONAL_RATE_ESTIMATE_CALC_FN required manually passing the closure to rate_with(...) on every call.

Key Changes

• Rate::peek: Non-mutating method returning estimated number of events observed in the current interval.
• Rate::proportional_rate: Ergonomic helper method returning real-time per-second rate estimation calculated over the sliding interval via PROPORTIONAL_RATE_ESTIMATE_CALC_FN.
• Unit Tests: Added test_peek_and_proportional_rate in pingora-limits/src/rate.rs.

Verification

• cargo test -p pingora-limits: Passed 9/9 unit tests cleanly.
• Signed-off-by: Yes.