asap-planner-rs: auto-infer metric labels from Prometheus #250

Problem

ControllerConfig requires a metrics field with explicit label names per metric. This is a manual step that blocks zero-friction deployment — users shouldn't need to know or specify label sets upfront.

Goal

When given a Prometheus URL, asap-planner-rs should query GET /api/v1/series?match[]=<metric_name> for each metric appearing in the query workload and automatically populate the MetricDefinition label lists, eliminating the need for a manually-specified metrics config.