Provide means to ignore warmup requests in performance tests

[kalantar]

Is your feature request related to a problem? Please describe.
In some cases (model serving with KServe, for example), there is a high cost to initial requests. When these are included in the results of the performance test, they bias the typical result.

Describe the solution you'd like
Enable a means to ignore some initial requests or requests for a period of time.

Additional context
fortio (HTTP tests) or ghz (gRPC) may provide mechanisms to do this.

Alternatively, it may be necessary to (internally) run the test for warmup and not record the results.