[Test] Increase registered callback's duration to avoid flakyness (#37243)

Previously the registered callback's duration is set too low (10ms) that 2 different OTel callbacks get called from OTel consecutively would trigger the registered callback again, making the test flaky.

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Closes #37243

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37243 from yijiem:otel-plugin-test-flake 70a1572cd2
PiperOrigin-RevId: 653717643

test/cpp/ext/otel/otel_plugin_test.cc

@@ -1919,7 +1919,7 @@ TEST_F(OpenTelemetryPluginCallbackMetricsTest,
         reporter.Report(double_gauge_handle, double_value_1++, kLabelValuesSet2,
                         kOptionalLabelValuesSet2);
       },
-      grpc_core::Duration::Milliseconds(10) * grpc_test_slowdown_factor(),
+      grpc_core::Duration::Milliseconds(50) * grpc_test_slowdown_factor(),
       integer_gauge_handle, double_gauge_handle);
   int report_count_2 = 0;
   int64_t int_value_2 = 1;
@@ -1936,9 +1936,9 @@ TEST_F(OpenTelemetryPluginCallbackMetricsTest,
         reporter.Report(double_gauge_handle, double_value_2++, kLabelValuesSet2,
                         kOptionalLabelValuesSet2);
       },
-      grpc_core::Duration::Milliseconds(10) * grpc_test_slowdown_factor(),
+      grpc_core::Duration::Milliseconds(50) * grpc_test_slowdown_factor(),
       integer_gauge_handle, double_gauge_handle);
-  constexpr int kIterations = 100;
+  constexpr int kIterations = 50;
   MetricsCollectorThread collector{
       this,
       grpc_core::Duration::Milliseconds(100) * grpc_test_slowdown_factor(),