stats/prometheus: normalize labels for single-label implementations

[vmg]

Description

Here's a bug we've found in production: when exporting Prometheus metrics, the name of the labels is not consistent when using Single-Label metrics vs Multi-Label metrics. More precisely, the Single-Label metrics API does not perform label normalization (i.e. from CamelCase to snake_case), while the Multi-Label API does perform this normalization.

The normalization into snake_case is sensible, as metric names in Prometheus are idiomatically written in snake case, so the lack of normalization when using single-label metrics feels like an oversight that caused our Prometheus metrics exports to report the wrong metrics.

After this change, the behavior between using a Single-Label metric and a Multi-Label metric with just one label is consistent. This is, however, a breaking change so it should be noted in the release notes.

Related Issue(s)

Fixes #12058

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[deepthi]

Ideally we would use assert/require but I see that the whole file is currently using t.Fatalf 😞