Change metrics to match generate_charts.py

[ballard26]

generate_charts.py takes the min of the publishLatencyMin and endToEndLatencyMin metrics in result.json. See https://github.com/redpanda-data/openmessaging-benchmark/blob/main/bin/generate_charts.py#L300 . Our validators for OMB results expect a single value for these metrics as well and not the list of values present in result.json. This PR changes our metrics validator to match this behavior and makes publishLatencyMin and endToEndLatencyMin a single value that is the min of their lists in result.json

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v23.3.x
• v23.2.x
• v23.1.x

Release Notes

• none