Variable substitution in Prometheus yaml file
UPDATE:
Looks like Thanos gives this out of the box.We have to configure Thanos to watch for the config file which has this templated variables and produce the output to a directory. And Prometheus then picks this file as its config file. This file would have the "substituted values" already from Thanos sidecar.
There is a standard way to do it after Prometheus 2.27.
--enable-feature=expand-external-labels
Replace ${var} or $var in the external_labels values according to the values of the current environment variables. References to undefined variables are replaced by the empty string.
And this reply by roidelaplui