Add sampling option to trace_analyzer

[zhichao-cao]

If the trace is very large or user want to sample the trace first based on the sampling ratio, user can set the sample_ratio to partially analyze the trace. It is based on every X query of one type, analyze one query. X = (1.0/sample_ratio).

Test plan: tested through the trace analyzing and test

[sagar0]

lgtm.

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