Fix default value overriding user-configured sampling ratio in distro (#44162)

* Fix default value overriding user-configured sampling ratio in distro

* Added CHANGELOG

* Improve readability

Author: rads-1996

sdk/monitor/azure-monitor-opentelemetry/CHANGELOG.md

@@ -8,6 +8,9 @@
 
 ### Bugs Fixed
 
+- Fix default value overriding user-configured sampling ratio in distro
+  ([#44162](https://github.com/Azure/azure-sdk-for-python/pull/44162))
+
 ### Other Changes
 
 ## 1.8.2 (2025-11-14)

sdk/monitor/azure-monitor-opentelemetry/azure/monitor/opentelemetry/_utils/configurations.py

@@ -187,15 +187,16 @@ def _default_sampling_ratio(configurations):
 
     # Handle all other cases (no sampler type specified or unsupported sampler type)
     else:
-        configurations[SAMPLING_RATIO_ARG] = default_value
+        if configurations.get(SAMPLING_RATIO_ARG) is None:
+            configurations[SAMPLING_RATIO_ARG] = default_value
         if sampler_type is not None:
             _logger.error(  # pylint: disable=C
                 "Invalid argument for the sampler to be used for tracing. "
                 "Supported values are %s and %s. Defaulting to %s: %s",
                 RATE_LIMITED_SAMPLER,
                 FIXED_PERCENTAGE_SAMPLER,
-                OTEL_TRACES_SAMPLER,
-                OTEL_TRACES_SAMPLER_ARG,
+                FIXED_PERCENTAGE_SAMPLER,
+                configurations[SAMPLING_RATIO_ARG],
             )
 
 def _default_instrumentation_options(configurations):

sdk/monitor/azure-monitor-opentelemetry/tests/utils/test_configurations.py

@@ -86,7 +86,7 @@ def test_get_configurations(self, resource_create_mock):
         self.assertEqual(configurations["resource"].attributes, TEST_MERGED_RESOURCE.attributes)
         self.assertEqual(environ[OTEL_EXPERIMENTAL_RESOURCE_DETECTORS], "azure_app_service,azure_vm")
         resource_create_mock.assert_called_once_with(TEST_CUSTOM_RESOURCE.attributes)
-        self.assertEqual(configurations["sampling_ratio"], 1.0)
+        self.assertEqual(configurations["sampling_ratio"], 0.5)
         self.assertEqual(configurations["credential"], "test_credential")
         self.assertEqual(
             configurations["instrumentation_options"],