SinkConnection::pending_requests_count

[rukai]

This PR exposes the pending_request count internally used within SinkConnection as part of its public API. In this PR I use it in the cassandra connection wrapper to demonstrate its functionality. But I intend to use it as part of the implementation of KafkaSinkConnection to handle lost connections to down kafka nodes.

Since its not clear whether this internal field is suitable for usage to track pending request count I added many test cases to ensure it behaves as expected.

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #1710 will not alter performance

_{Comparing rukai:sink_connection_pending_requests (f4ab381) with main (9dd04c1)}

Summary

✅ 39 untouched benchmarks