[tests] Fix `fake_statsd_server.py` received metric counting

[sgnn7]

What does this PR do?

Fixes counting of received metrics in manual test modules

Motivation

Since the introduction of newlines to the end of all sent statsd messages, our fake_statsd_server and statsd_throughput test modules have been miscounting the metrics received. While this has no impact on the users, we need the accurate numbers in manually-run tests.

Alternate Designs

N/A

Possible Drawbacks

N/A

Verification Process

• P=3 BENCHMARK_TRANSPORT="UDP" BENCHMARK_NUM_THREADS=10 BENCHMARK_NUM_RUNS=5 BENCHMARK_NUM_DATAPOINTS=5 "python${P}" -m unittest -vvv tests.performance.test_statsd_throughput
• Ensure that metrics received percentage is close to 100% rather than 200%

Additional Notes

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Release Notes

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Review checklist (to be filled by reviewers)

• [ ] Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• [ ] PR title must be written as a CHANGELOG entry (see why)
• [ ] Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• [ ] PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
• [ ] PR should not have do-not-merge/ label attached.
• [ ] If Applicable, issue must have kind/ and severity/ labels attached at least.