As announced last week there is now a Matomo Tag Manager available which integrates into Matomo and eventually will be directly shipped with Matomo.

It would be great if you could offer an easy way for users to embed their Tag Manager containers. We have written a guide that helps on how to write an integration plugin:
https://developer.matomo.org/guides/tagmanager/integration-plugin

Please let me know if there are any questions. Happy to improve the guide and making integrations as easy as possible.

FYI: Piwik is now Matomo.