fix: cap percentile at last finite bucket when target falls in open +Inf bucket

plugins/traefik/scraper.py

@@ -8,6 +8,7 @@ Traefik exposes Prometheus metrics at /metrics. This module:
    and latency percentiles (p50, p95, p99) via linear histogram interpolation
 """
 from __future__ import annotations
+import math
 import re
 from collections import defaultdict
 
@@ -171,10 +172,14 @@ def _percentile_ms(
         prev_le, prev_count = router_buckets[i - 1]
         if count == prev_count:
             return prev_le * 1000.0
+        if math.isinf(le):
+            # Percentile falls in the open (last_finite_le, +Inf] bucket;
+            # cap at the last finite bucket upper bound.
+            return prev_le * 1000.0
         fraction = (target - prev_count) / (count - prev_count)
         return (prev_le + fraction * (le - prev_le)) * 1000.0
 
-    return router_buckets[-2][0] * 1000.0  # last finite bucket upper bound
+    return 0.0  # unreachable: +Inf bucket always satisfies count >= target
 
 
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