compliance: add OpenMetrics 1.0 vs 2.0 e2e scrape benchmark

[bwplotka]

Adds a //go:build demo e2e scrape benchmark in compliance/openmetrics_scrape_bench_test.go using efficientgo/e2e and e2emon on top of prometheus#19880.

The benchmark spins up side-by-side prom-om1 (OpenMetricsText1.0.0) and prom-om2 (OpenMetricsText2.0.0) instances scraping:

• 25 client_golang targets (1,000 metric series per target: 400 counters, 400 gauges, 100 classic histograms, 100 summaries, with exemplars and created timestamps).
• 25 virtual targets against isolated client_java 1.9.0 Docker containers (europe-west3-docker.pkg.dev/macro-mile-203600/test/openmetrics2-java-demo:1.9.0).

Run from compliance/:

BENCH_DURATION=30s go test -v -tags=demo -timeout 10m -run '^TestBenchmarkOpenMetricsScrape$' .
BENCH_NHCB=true BENCH_DURATION=30s go test -v -tags=demo -timeout 10m -run '^TestBenchmarkOpenMetricsScrape$' .

NONE