[metrics] Fix test flakiness (#38128)

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Closes #38128

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/38128 from yashykt:TestMetricstestFlakiness 4ac65b2d80
PiperOrigin-RevId: 697000317

test/core/telemetry/metrics_test.cc

@@ -669,6 +669,8 @@ TEST_F(MetricsTest, ParallelStatsPluginRegistrationAndLookup) {
       while (GlobalStatsPluginRegistry::GetStatsPluginsForChannel(
                  StatsPluginChannelScope("", "", endpoint_config_))
                  .size() < 10000) {
+        // Yield to avoid starving the register threads.
+        std::this_thread::yield();
       };
     });
   }