benchmarks(agentic): restore generate_aiperf_plots.py for server-metrics panels

benchmark_lib.sh:1052 still invokes utils/generate_aiperf_plots.py to
produce metrics_plots.png after each agentic run, but the script was
removed alongside the pareto-frontier helpers in ed399bf and the
caller's "|| true" silently swallowed the file-not-found error. Result:
no metrics_plots.png in any agentic artifact for the past 5 days.

Restore the script verbatim from ed399bf^. The 6x2 panel covers KV
cache utilization, queue depth, prefix-cache hit rate, throughput, KV
offload transfer + cumulative GB, TTFT/latency/ITL scatters, and
preemptions — none of which are pareto frontiers. Keep the pareto/
sweep-overview deletions from ed399bf in place.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Signed-off-by: Cam Quilici <cjquilici@gmail.com>

Author: cquil11