feat(prometheus): widget template format, checker and renderer (#5690)

#5690 "Widget template format: YAML schema, validator and renderer", step 2
of milestone 508 "Prometheus widget templates".

- template_format.check(text) -> Template, or TemplateError with every
  problem, each with its line and what to change. Blocks: states, values
  (of -> share and bar, thresholds as numbers or shares, sparkline),
  times (warn_after / crit_after), table (two labels pivoted), info.
  Anchors/aliases refused, 64 KB cap, YAML's on/off/yes/no flagged.
- template_view.render(template, target, latest, now, history) -> View:
  a verdict naming the worst problem, then each block with its problems
  first (crit, warn, ok, then quiet states). Missing series show as "—"
  and say so; a template whose `expects` prefix the target lacks says so.
- shown_value moves to service.py so the renderer can use it.

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

plugins/prometheus/service.py

@@ -5,11 +5,12 @@ from __future__ import annotations
 
 import math
 from dataclasses import dataclass, field
-from datetime import datetime
+from datetime import datetime, timezone
 from fnmatch import fnmatchcase
 
 from steward.core import prometheus
 from steward.core.crypto import encrypt_secret
+from steward.core.formatting import metric_value
 
 from .models import PrometheusTarget
 
@@ -121,6 +122,19 @@ def fold_states(metric: str, rows: list[dict]) -> list[dict] | None:
     return folded
 
 
+def shown_value(metric: str, value: float | None) -> dict:
+    """A series' value in its unit, from the Prometheus naming conventions.
+    A *_timestamp_seconds is a moment: it comes back as `moment` for the page
+    to show as "3m ago" in the reader's zone; 0 there means "never"."""
+    if value is None:
+        return {"text": "—", "moment": None}
+    if metric.endswith("_timestamp_seconds"):
+        if value <= 0:
+            return {"text": "never", "moment": None}
+        return {"text": "", "moment": datetime.fromtimestamp(value, timezone.utc)}
+    return {"text": metric_value(metric, value), "moment": None}
+
+
 @dataclass
 class Series:
     resource: str