exporter/prometheus: replace Quantile proto comparisons by map (#2924)
exporter/prometheus: replace Quantile proto comparisons by map (#2924)

Previously we were plainly comparing slices of proto Quantile values,
but there were flakes due to ordering and some random extra values.

This change just extracts those quantile values into a
map[float64]float64 for direct comparisons.

Fixes #2922
open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector-contrib,open-telemetry/opentelemetry-collector-contrib,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector