feat(sdk): Add automatic rate computation for read/write metrics

[lowhung]

Summary

This PR adds automatic rate computation (messages/second) to the SDK.

Changes

• Track previous count and timestamp for each topic in ReadState and WriteState
• Compute messages/second rate on each collect() call
• Rate is None on first collection (no baseline available)
• Requires at least 10ms between samples to avoid division issues

Testing

• Added comprehensive tests for rate computation
• Tests verify rate is None on first collection
• Tests verify rate is computed correctly on subsequent collections
• Tests verify edge cases (zero delta, short elapsed time)

Usage

let instrumentor = Instrumentor::new();
let handle = instrumentor.register("my-service");

// Record some messages
handle.record_read("events", 100);

// First collection - rate will be None
let snapshot1 = instrumentor.collect();

// Wait and record more
std::thread::sleep(Duration::from_secs(1));
handle.record_read("events", 100);

// Second collection - rate will be ~100 msg/s
let snapshot2 = instrumentor.collect();