add apinote annotation

Signed-off-by: Jay DeLuca <jaydeluca4@gmail.com>

Author: jaydeluca

prometheus-metrics-config/src/main/java/io/prometheus/metrics/config/OpenMetrics2Properties.java

@@ -3,8 +3,10 @@
 import javax.annotation.Nullable;
 
 /**
- * Properties starting with io.prometheus.openmetrics2. These properties are experimental and
- * subject to change.
+ * Properties starting with io.prometheus.openmetrics2.
+ *
+ * @apiNote
+ * These properties are experimental and subject to change.
  */
 public class OpenMetrics2Properties {