[BUILD] Only build and run the exemplar tests if the preview flag is set (open-telemetry#3675)

Author: dbarker

sdk/test/metrics/CMakeLists.txt

@@ -80,4 +80,6 @@ if(WITH_BENCHMARK)
     metrics_common_test_utils opentelemetry_common opentelemetry_resources)
 endif()
 
-add_subdirectory(exemplar)
+if(WITH_METRICS_EXEMPLAR_PREVIEW)
+  add_subdirectory(exemplar)
+endif()