title: detect elastic search index last modified time
body: <p>I'm wondering what would be an efficient way to detect the last modified timestamp of an index in Elastic Search. I have read posts of adding a timestamp fields in pipeline but this solution has limitations (e.g. only newly created index supports timestamp update?)</p>
  <p>If only a handful of indices are required to track their last modify time, what would be the most efficient way? Would periodic query and compare result between queries give us an approx. last modify time? any other ways to track ES events?</p>