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>

docs/plugins/prometheus.md

@@ -0,0 +1,84 @@
+# Prometheus Plugin
+
+The Prometheus plugin scrapes any service's Prometheus `/metrics` endpoint and stores what you choose to keep in Steward's metrics. Once stored, a series can be charted, read through the MCP `query_metrics` tool, and watched by an alert rule like any other metric.
+
+It is on by default. Targets are added, tested and edited on its page, **Prometheus** in the sidebar (`/plugins/prometheus/`).
+
+---
+
+## Adding a target
+
+| Field | What it is |
+|---|---|
+| Name | How the target's series are named in Steward (rule pickers, charts). No `/`, `:` or comma. |
+| Metrics address | The full URL, e.g. `http://archives:8081/metrics`. |
+| Bearer token | Sent as `Authorization: Bearer …`. Stored encrypted and never shown again; leave it blank to keep the saved one. |
+| Scrape every | 10 s to an hour. Default 60 s. |
+| Check its HTTPS certificate | Off only for a self-signed certificate you trust. |
+| Keep / Leave out | Name patterns, one a line, `*` for any run of characters. Empty Keep keeps everything; Leave out wins over Keep. |
+| Series cap | The most series stored from one scrape (default 1000). Past it, the rest are counted and shown on the target's page, not stored. |
+
+**Test** scrapes the address once with what is typed, filters included, and says how many series would be stored and under which names. Nothing is saved.
+
+The endpoint hands over everything; Steward chooses what to keep. Start narrow (`yourservice_*`) and widen it if you find you want more: every stored series is a row every scrape.
+
+---
+
+## How series are stored
+
+| On the endpoint | In Steward |
+|---|---|
+| A gauge or an untyped sample | Stored as it is, under its own name. |
+| A counter (`requests_total`) | Stored as a rate per second, `requests_per_sec`, from the second scrape on. A value lower than the last (the service restarted) is skipped rather than read as a negative rate. |
+| A histogram or summary | Its `_sum` and `_count` are stored as rates (`x_sum_per_sec`, `x_count_per_sec`). Buckets and quantiles are not stored. |
+| NaN or ±Inf | Not stored. |
+
+A series without labels is stored under the target's name. A labelled one is stored under `<target>:<label>=<value>,…`, in the order the endpoint gives the labels, for example `Archives:target=Steward,state=late`.
+
+Each target also records four metrics about itself, under its own name:
+
+| Metric | Meaning |
+|---|---|
+| `is_up` | 1 when the last scrape got metrics, 0 when it didn't. |
+| `scrape_ms` | How long the scrape took. |
+| `series_kept` | Series stored from the last scrape. |
+| `series_capped` | Series the last scrape left out because of the cap. |
+
+A value's unit is read from Prometheus naming: `_bytes` as bytes, `_seconds` as a span, `_timestamp_seconds` as a moment ("3h ago"), `_per_sec` as a rate.
+
+Renaming a target moves its stored history to the new name. Deleting one removes its newest values; its history ages out with retention (Settings → Retention).
+
+---
+
+## Alerting on a target
+
+In **Alerts → New rule**, choose **Prometheus targets**, then the series (the resource) and the metric. Two rules worth having for any target:
+
+- **It stopped answering:** resource `<target>`, metric `is_up`, `< 1`.
+- **Something was cut off:** resource `<target>`, metric `series_capped`, `> 0`.
+
+---
+
+## Recipe: Fabled Archives
+
+Archives serves `/metrics` on its backup listener (port 8081), which is reachable only on your LAN and the swarm's overlay network, behind a scrape token.
+
+1. In Archives, **Settings → Metrics → Make a token**, named e.g. "Steward". Copy it; it is shown once.
+2. In Steward, **Prometheus → Add target**:
+   - Name `Archives`
+   - Metrics address `http://<archives host>:8081/metrics` (on a shared overlay network, the service name, e.g. `http://archives_app:8081/metrics`)
+   - Bearer token: the one you copied
+   - Keep: `fabledarchives_*`
+3. **Test**, then **Add target**.
+
+Rules worth adding:
+
+| Watch | Resource | Metric | Condition |
+|---|---|---|---|
+| A backup target is late | `Archives:target=<name>,target_id=<id>,state=late` | `fabledarchives_backup_target_state` | `>= 1` |
+| A backup target is missing | `Archives:target=<name>,target_id=<id>,state=missing` | `fabledarchives_backup_target_state` | `>= 1` |
+| A replication job is failing | `Archives:job=<name>,job_id=<id>,state=failing` | `fabledarchives_replication_job_state` | `>= 1` |
+| Archives' data disk is filling | `Archives` | `fabledarchives_data_disk_free_bytes` | `<` your floor |
+| Archives stopped answering | `Archives` | `is_up` | `< 1` |
+
+The target's page lists every stored series with its exact resource name, so copy names from there rather than typing them.