Add tool: prometheus_list_metric_series (sample label sets for a metric)

[brenoepics]

ID: tool.metric_series_sample
Priority: P2
Component: mcp/tools + repository

Given a metric name and optional matchers, return a deduped sample of label sets using /api/v1/series.

Acceptance Criteria

• Input: {metric: string, matchers?: string[], limit?: number}
• Output: array of label maps
• Repository reuses get_series with selector construction
• Test ensures limit is enforced

Estimate: 2