Telegraf has [something similar]( https://github.com/influxdata/telegraf/blob/master/docs/CONFIGURATION.md#environment-variables
), but there are limitations. Most notably that it isn't possible to assign an array of strings using it.
We have considered adding more functionality like this, but are holding off for now. I do recommend using a templating library, perhaps the one that comes with your configuration management software.
Personally, I'm using ansible and using it's built-in Jinja templating provides better integration than I would have if we added a template library directly to Telegraf itself. I don't usually use the `lineinfile` module, which is similar to your regexp editing, and instead stick to full plugins as the smallest unit. Merging the top level telegraf.conf isn't much of a problem for me either, since I only use it for the agent table. In the next major release of Telegraf, we may do this split as default, but for now we don't want to upend anyone's process.
Hope that adds a little color to the current setup.