[Nexthop] Use timestamped port stats for line rate calculation for getTrafficRate

[vvasavada-nexthop]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

Summary

Port stats are sampled asynchronously by the stats thread, so the real interval between two samples can differ from the request duration. Use actual timestamped delta when available.

This fix is similar to #1505 but done in getTrafficRate function used by AgentTrafficPauseTest.*.

Test Plan

[ PASSED ] cold_boot.AgentTrafficPauseTest.verifyPauseRxOnly (74549 ms)
[ PASSED ] warm_boot.AgentTrafficPauseTest.verifyPauseRxOnly (47263 ms)
[ PASSED ] cold_boot.AgentTrafficPauseTest.verifyPauseTxOnly (70542 ms)
[ PASSED ] warm_boot.AgentTrafficPauseTest.verifyPauseTxOnly (62234 ms)
[ PASSED ] cold_boot.AgentTrafficPauseTest.verifyPauseRxTx (66189 ms)
[ PASSED ] warm_boot.AgentTrafficPauseTest.verifyPauseRxTx (47644 ms)