Ensure all versions of Content Type are correctly handled in the code.

[ishleenk17]

The Prometheus metricset currently supports only version 0.0.4 of the plain.text content type.
Add support for version 0.0.1 as well. Currently if this version is used, it actually doesn't parse this version and fallback to the fallback scrape protocol.
Also ensure that the content types are actually similar to the Prometheus Content Types.

[botelastic]

This issue doesn't have a Team:<team> label.