code: @JsonIgnore
      public String getTag(String key) {
          return (!Metric.ReservedField.isReservedField(key)) ? _tags.get(key) : null;
      }
docs: Returns the tag value for the given key.
  
  @param   key  The tag key. May be null.
  
  @return  The value of the tag, or null if no value exists.