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

[lowhung]

Summary

The SDK's ReadMetrics and WriteMetrics have a rate field but it's always None. The SDK should compute messages/second automatically.

Proposed Solution

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

Benefits

• Real-time throughput visibility without manual calculation
• Consistent rate computation across all topics

Implementation Notes

Already implemented in PR #8

[lowhung]

PR #8 has been merged. This issue can now be closed as the automatic rate computation feature is complete.

The SDK now tracks previous count and timestamp for each topic and computes messages/second rate on each collect() call, with a minimum 10ms sample interval to avoid division issues.

[lowhung]

Closing as implemented in PR #8.