feat(core): one Prometheus text parser, with types; Traefik reads through it (#5667)

steward/core/prometheus.py parses the 0.0.4 exposition into families keyed
by name, each with its declared TYPE, so a scraper can tell a counter from a
gauge. A histogram or summary owns its _bucket/_sum/_count samples. Label
values are unescaped, and commas or braces inside quotes stay part of the
value. NaN and +/-Inf are read, timestamps are ignored, and a malformed
line is skipped. Traefik's parse_prometheus is now a thin reshaping of it.

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

plugins/traefik/scraper.py

@@ -18,6 +18,8 @@ from datetime import datetime, timezone
 
 import httpx
 
+from steward.core import prometheus
+
 
 # ──────────────────────────────────────────────────────────────────────────────
 # Prometheus text format parser
@@ -26,36 +28,14 @@ import httpx
 # Parsed sample: {metric_name: {frozenset(label_items): float}}
 ParsedMetrics = dict[str, dict[frozenset, float]]
 
-_SAMPLE_RE = re.compile(
-    r'^([a-zA-Z_:][a-zA-Z0-9_:]*)(?:\{([^}]*)\})?\s+'
-    r'([+-]?(?:\d+\.?\d*|\.\d+)(?:[eE][+-]?\d+)?|[+-]?Inf|NaN)'
-)
-_LABEL_RE = re.compile(r'([a-zA-Z_][a-zA-Z0-9_]*)="([^"\\]*(?:\\.[^"\\]*)*)"')
-
 
 def parse_prometheus(text: str) -> ParsedMetrics:
-    """Parse Prometheus text exposition format into nested dicts.
-
-    Returns: {metric_name: {frozenset({(label, value), ...}): float}}
-    """
+    """The exposition keyed by sample name, then by its label set — the shape
+    the per-router arithmetic below reads. Parsing is steward.core.prometheus."""
     metrics: ParsedMetrics = {}
-    for line in text.splitlines():
-        line = line.strip()
-        if not line or line.startswith("#"):
-            continue
-        m = _SAMPLE_RE.match(line)
-        if not m:
-            continue
-        name, labels_str, value_str = m.groups()
-        try:
-            value = float(value_str)
-        except ValueError:
-            continue
-        labels = frozenset(
-            (lm.group(1), lm.group(2))
-            for lm in _LABEL_RE.finditer(labels_str or "")
-        )
-        metrics.setdefault(name, {})[labels] = value
+    for family in prometheus.parse(text).values():
+        for sample in family.samples:
+            metrics.setdefault(sample.name, {})[frozenset(sample.labels)] = sample.value
     return metrics