Fix throughput class creation bug

Parameter start_ts in function create_throughput_measurement is defined by default as start_ts=time.perf_counter(). In python this definition will lead to the start initialization of the parameter. If we wil call this method in further without the parameter, start_ts will be equal to the first value.

Acceptance Criteria

• Move the definition of the parameter start_ts to the function body.
• Add test