Don't see "X-Instance-Identity" (Jenkins instance id) in the response header

Question

It is documented that each Jenkins instance has a unique id that we can retrieve by doing a GET. I am unable to find it. For e.g. do a GET on https://builds.apache.org/api/json and see the response headers

Answer 1

The https://builds.apache.org/ URL has it. Emphasis mine:

the public key can be obtained by sending the GET request to the top page of Jenkins, and look for the "X-Instance-Identity" header in the response