DTM was never really made to deploy meta tags. While the solution Parit provided may work in some cases, it's not perfect. The reason has to do with the way the page loads. DTM is a script that runs typically after the browser has already parsed through the meta tags on the page. DTM is then going back and writing that meta tag into the header via the document.createElement. At this point, the browser has likely already moved passed the meta tag processing.
As you indicated, most services like to have the meta tag included as part of the static source code so they can guarantee it will be there before the browser starts to parse through the page.
Overall, I wouldn't recommend deploying meta tags via any tag manager solution, including DTM. A quick search on your favorite search engine for "deploy meta tags via tag manager" will result in a number of people testing this with unstable results at best.