Constant METRIC_SINK_PREFIX_MARKER 

const METRIC_SINK_PREFIX_MARKER: &str = "mz_metric_sink_";

The reserved namespace every metric sink prefix must start with. It confines the family names a sink publishes (prefix + name) to the mz_metric_sink_ lane, which nothing else in a replica’s Prometheus registry writes: platform metrics use other mz_* names, third-party collectors namespace away from mz_*, and the sink’s own health gauges are mz_compute_metric_sink_*. Requiring the marker rather than blocklisting platform names cannot rot as new platform collectors are added.