fix: safe label extraction for Prometheus-style metric keys

Replace unsafe chained-split parsing of metric labels in the
cache performance monitoring section with a reusable static
helper method .

The old code:
  cache_type = metric_name.split('cache_type=')[1].split(',')[0].split('}')[0]
risked IndexError if the guard at line 477 was removed or
refactored. The new method safely returns None when the label
is not found, and correctly extracts the value up to the next
 or  delimiter.

Fixes #873

trustgraph-flow/trustgraph/query/ontology/monitoring.py

@@ -7,6 +7,7 @@ import logging
 import time
 import asyncio
 import inspect
+import re
 import threading
 from typing import Dict, Any, List, Optional, Callable
 from dataclasses import dataclass, field
@@ -275,6 +276,23 @@ class MetricsCollector:
         label_str = ','.join(f"{k}={v}" for k, v in sorted(labels.items()))
         return f"{name}{{{label_str}}}"
 
+    @staticmethod
+    def _extract_label(metric_key: str, label: str) -> Optional[str]:
+        """Extract a label value from a metric key built by _build_key.
+
+        Metric keys follow the format: name{key1=val1,key2=val2}
+        Returns None if the label is not found.
+        """
+        prefix = f"{label}="
+        if prefix not in metric_key:
+            return None
+        # Find the label= then get value until , or }
+        after_label = metric_key.split(prefix, 1)[1]
+        for delim in (',', '}'):
+            if delim in after_label:
+                return after_label.split(delim, 1)[0]
+        return after_label
+
 
 class PerformanceMonitor:
     """Monitors system performance and component health."""
@@ -474,8 +492,8 @@ class PerformanceMonitor:
         # Cache performance
         cache_types = set()
         for metric_name in self.metrics_collector.counters.keys():
-            if 'cache_type=' in metric_name:
-                cache_type = metric_name.split('cache_type=')[1].split(',')[0].split('}')[0]
+            cache_type = self.metrics_collector._extract_label(metric_name, 'cache_type')
+            if cache_type is not None:
                 cache_types.add(cache_type)
 
         for cache_type in cache_types: