Constant METRIC_NAME_PATTERN 

const METRIC_NAME_PATTERN: &str = "^[a-zA-Z_:][a-zA-Z0-9_:]*$";

Matches Prometheus’s metric name grammar: [a-zA-Z_:][a-zA-Z0-9_:]*.

Expressed in MIR (see shape_metric_sink_source) rather than parsed from a &str on the operator’s hot path.