bechmark: scale up RPS to test data plane CPU performance

[zhaohuabing]

What type of PR is this?
This PR adjusts the Envoy Gateway benchmark to measure data plane CPU performance using RPS instead of route count.

While route count is useful for evaluating the CPU performance patterns of the control plane(EG) , data plane(Envoy) CPU usage is primarily driven by request processing. Scaling load by RPS provides a more stable signal, and makes the results easier to interpret for data plane performance analysis.

It also fixes a bug in the current sampling logic where the sample data is set to 0 when failed to query Prometheus, now the failed sampling data is not considered when calculating the min/max/men in the report.

The current bechmark report:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.63%. Comparing base (a26fba5) to head (ad6959e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7810      +/-   ##
==========================================
- Coverage   72.63%   72.63%   -0.01%     
==========================================
  Files         235      235              
  Lines       34873    34873              
==========================================
- Hits        25330    25329       -1     
- Misses       7739     7740       +1     
  Partials     1804     1804              

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