feat: add initial load test

[ryanzhang-oss]

Description of your changes

Add a load test to measure the client-side latency for placement operations.

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How has this code been tested

Special notes for your reviewer

I have run the test against fleet but hit a guard issue that prevent it from running more than 20 in parallel

[Fei-Guo]

One suggestion, can we specify the test length (e.g, the total number of created CRP crs) and report the metrics result in the end. Hence the test can be used as a benchmark.

[ryanzhang-oss]

One suggestion, can we specify the test length (e.g, the total number of created CRP crs) and report the metrics result in the end. Hence the test can be used as a benchmark.

Good idea to have a test length. It turned out that getting metrics with Prometheus client go lib is not easy (especially with the histogram). It's a lot easier to curl the metrics endpoint.

[Fei-Guo]

I would suggest adding a few atomic_int variables to count the total number of CRP crs, the number of timeout crs, etc, and print out the result when the program exits, we don't need to dump the histogram. This should good enough for finding some critical issues.