Fix parametric otel_metrics tests leaking runtime metrics

Add DD_RUNTIME_METRICS_ENABLED=false to DEFAULT_ENVVARS to prevent
System.Runtime and Microsoft.AspNetCore.* metrics from polluting
custom metric test assertions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: link04

tests/parametric/test_otel_metrics.py

@@ -43,6 +43,7 @@
 #   CORECLR_ENABLE_PROFILING=1 is required in .NET to enable auto-instrumentation
 DEFAULT_ENVVARS = {
     "DD_METRICS_OTEL_ENABLED": "true",
+    "DD_RUNTIME_METRICS_ENABLED": "false",  # Prevent runtime metrics (System.Runtime, Microsoft.AspNetCore.*) from leaking into custom metric tests
     "OTEL_METRIC_EXPORT_INTERVAL": "60000",  # Mitigate test flake by increasing the interval so that the only time new metrics are exported are when we manually flush them
     "CORECLR_ENABLE_PROFILING": "1",
 }