docs: add Metrics page for the new optional Prometheus support New Metrics.md covers enabling/auth, the fork-safety design (mirroring Scheduling.md's own multi-worker writeup), the full metric reference table, and cardinality rationale for action_type-only labels. Linked from Home.md/_Sidebar.md, cross-referenced from HTTP-API-Reference.md (JSON vs text-exposition routes), Architecture-Internals.md (module map + fork-safety note), Security.md (auth model, what's exposed), and Scheduling.md (reciprocal link -- same category of fork-safety problem, different solution).