rttanalysis: avoid tracing during non-validation benchmark

[rafiss]

The benchmarks in this package were all capturing statement traces. That is useful for when we want to measure the number of KV roundtrips and compare it to the expected number, but when we run the test cases as normal benchmarks the overhead of capturing a trace distorts all measurements.

Now we only capture traces when we need to, in the TestBenchmarkExpectation test.

Epic: None
Release note: None

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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[cockroach-teamcity]

This change is