Allow for merging of Text outputs

[abergmeier]

Is your feature request related to a problem? Please describe.
Due to Prometheus' limitations in scraping it often is necessary to merge metrics of different endpoints and expose as one endpoint.
For this merging of Text outputs would be necessary.

Describe the solution you'd like
I think the biggest hindrance is that there is not yet a Decoder implemented.
Software that does already do the merging has been implemented here: https://github.com/QubitProducts/exporter_exporter

Describe alternatives you've considered
Alternative would be to fix scraping logic. With years of discussion behind that I think the path of least resistance is doing the merging on the client side.