Adding span builder reuse variable (#9839)

Author: dougqh

metadata/supported-configurations.json

@@ -439,6 +439,7 @@
     "DD_SERVICE_NAME": ["A"],
     "DD_SERVICE_NAME_SET_BY_USER": ["A"],
     "DD_SITE": ["A"],
+    "DD_SPAN_BUILDER_REUSE_ENABLED": ["A"],
     "DD_SPAN_SAMPLING_RULES": ["A"],
     "DD_SPAN_SAMPLING_RULES_FILE": ["A"],
     "DD_SPARK_APP_NAME_AS_SERVICE": ["A"],