Provide a default for missing values in KeyValue

fixes micrometer-metricsgh-3458

Author: jonatan-ivanov

micrometer-commons/src/main/java/io/micrometer/common/KeyValue.java

@@ -28,6 +28,11 @@
  */
 public interface KeyValue extends Comparable<KeyValue> {
 
+    /**
+     * Use this if you want to indicate that the value is missing.
+     */
+    public static final String NONE = "none";
+
     String getKey();
 
     String getValue();