This fixes issues with certain metrics scrapers, which might error if the content-type is not correctly set.
## Issue Addressed
Fixes https://github.com/sigp/lighthouse/issues/3437
## Proposed Changes
Simply set header: `Content-Type: text/plain` on metrics server response. Seems like the errored branch does this correctly already.
## Additional Info
This is needed also to enable influx-db metric scraping which work very nicely with Geth.