User-defined metadata for Merge Request? Or how to pass variables to the job with MR?

Hi all,
Is it possible to specify a variable within Merge Request (MR) that will be available during the job execution?

My usecase is quite simple, I have a big REST API system to test, with a lot of tests covering different areas of the product. What I want to achieve is to pass a variable within a MR that will be used inside the job to run a subset of tests, defined by this variable.

For example if a MR title has [CI_USERTAG: DHCP] a test launcher inside the job will run the tests related to DHCP subsystem only, skipping all unrelated tests.

Is it possible to achieve this somehow?