fix(prometheus): the list page is a section top; the scrape test builds its text without format(); docs (#5668, #5670)

- tests/test_breadcrumbs.py: the Prometheus target list is its section's
  own top page, like SNMP's, so it carries no trail.
- The integration test's exposition has label braces, which str.format read
  as fields: the fake scrape raised, the target was recorded down and no
  series landed. It substitutes the counter with replace() now.
- docs/plugins/prometheus.md: adding a target, filtering, how series are
  named and stored, alerting, and the Fabled Archives recipe.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -122,4 +122,5 @@ See `docs/plugins/` for a full plugin development guide.
 | `docs/plugins/overview.md` | Plugin system: how loading works, plugin.yaml schema, required exports |
 | `docs/plugins/writing-a-plugin.md` | Step-by-step plugin development guide |
 | `docs/plugins/traefik.md` | Traefik plugin: config, metrics, alert rule examples |
+| `docs/plugins/prometheus.md` | Prometheus plugin: scraping any /metrics endpoint, how series are stored, the Fabled Archives recipe |
 | `docs/reference/code-map.md` | Where every key function, model, and route lives in the codebase |