feat: support info metrics

[bernot-dev]

Add support for INFO metrics by converting them to Gauges.

[bwplotka]

That's generally should be it, but also test will tell us more.

I'd recommend adding one to https://github.com/GoogleCloudPlatform/prometheus/blob/release-2.53.5-gmp/google/internal/promqle2etest/gcm_test.go

Since SDK does not support info metrics you might need some slight change in how we setup test framework to inject custom text format 🤔 that might be the main work here. If I were you I would attempt to extend (you are welcome to contribute too!) https://github.com/prometheus/compliance/blob/7667b56d892fbb4f4252287e50bbad6d2a8f6893/promqle2e/promqle2e.go#L435 so it accepts []string with OpenMetrics raw TEXT to show (and change) on every scrape and expect some result.

This would benefit the promqle2e and our environment, but if you find a different way to test this (e.g. extending our side local export GCM helper:

prometheus/google/internal/promqle2etest/backend_export_gcm.go

Line 158 in 3a83a91

func (l *runningLocalExportWithGCM) IngestSamples(ctx context.Context, t testing.TB, recorded [][]*dto.MetricFamily) {

)

[bwplotka]

Thanks.

LGTM, modulo tiny commentary nits (keep it a full English sentence, please).

I wished we could test it e2e with GCM, but this is good enough, we can do e2e test later and manually deploy latest GMP with KSM with info metric.

Added: #281

[bwplotka]

NOTE: Same work is needed for stateset, but low prio.