Add registry option to skip metric sorting

[ibobgunardi]

Fixes #483.

Adds RegistryOpts.DisableMetricSorting with option-aware constructors for vanilla and pedantic registries. The existing constructors keep the current sorted output; the new option skips the final MetricFamily/Metric sort for callers that can accept unspecified ordering.

Tests:

• go test ./prometheus -run TestRegistryDisableMetricSorting -count=1
• go test ./prometheus -run 'TestRegistry|TestRegisterUnregisterCollector|TestCollectorOnMetricPanic|TestCheckMetricConsistency|TestGatherDoesNotLeakGoroutines' -count=1
• go test ./prometheus/internal -count=1

Note: go test ./prometheus -count=1 fails locally on Go 1.25.1 because the expected Go runtime metric set has 180 entries while this toolchain exposes 177.

[kakkoyun]

LGTM.

Really solid PR.

[kakkoyun]

I will wait for another maintainers' opinion before merging it @vesari @bwplotka

[bwplotka]

Thanks for this, solid PR, but we add bunch of new APIs and increase amount of options.

Do we know, for what exactly benefit? Can we add a quick micro benchmark to see how much we would really save in terms of time/memory?

The issue is #483 old - notably KSM moved to a hand coded exposition, so we don't need to support it? Curious how much benefit we have for 100 metric families vs 10k metric families.