def with_tag(key, value)
      new_tags = @tags.dup
      new_tags[key] = value
      MeterId.new(@name, new_tags)
    end
def with_tag(key, value)
      new_tags = @tags.dup
      new_tags[key] = value
      MeterId.new(@name, new_tags)
    end
Create a new MeterId with a given key and value