Add --validation-scheme flag for UTF-8 metric name support

[friedrichg]

Summary

The Prometheus rulefmt.Rule.Validate() method now accepts a model.ValidationScheme parameter that controls whether metric and label names must follow legacy Prometheus naming rules or can be any valid UTF-8 string.

Currently cortex-tools hardcodes model.LegacyValidation. As Cortex and Prometheus adopt UTF-8 metric name support, users should be able to control this.

Proposal

Add a --validation-scheme flag (default legacy) to rule-related commands:

• rules check
• rules lint
• rules load

Accepted values:

• legacy — metric/label names must match [a-zA-Z_:][a-zA-Z0-9_:]*
• utf8 — only requires valid UTF-8 strings

References

• Prometheus UTF-8 metric names: https://prometheus.io/docs/concepts/data_model/#metric-names-and-labels
• model.ValidationScheme in github.com/prometheus/common/model
• Current hardcoded usage: pkg/rules/rules.go in ValidateRuleGroup()