/metrics endpoints should return atleast text/plain content-type so browser knows it can display instead of downloading

# What is currently missing?
The prometheus seems to be robust enough to handle it regardless, but when you access the metrics endpoint from a browser it will return text but the content-type sent from the server is `application/octet-stream` at minimum it should return `text/plain` or more if you want to be exact `text/plain; version=0.0.4; charset=utf-8;`
(PromtheusText0.0.4).

Reference:
https://prometheus.io/docs/instrumenting/content_negotiation/

# Is this a feature you would work on yourself?
Unfortunetly I do not have the time at the moment.