Add tool: prometheus_metric_label_values (label values scoped to metric) #20
Description
ID: tool.metric_label_values
Priority: P2
Component: mcp/tools + repository
For a metric and label, compute values by querying /api/v1/series with a selector, avoiding full label scan.
Acceptance Criteria
- Input: {metric: string, label: string, matchers?: string[], start?: string, end?: string}
- Efficient call path using series + filter
- Tests with fixture data
Estimate: 2