Adjust Content-Type header for Prometheus endpoint to include version

Adjust Content-Type header for Prometheus endpoint to include version

Prometheus v3 will, by default, be more strict about the content-types
returned from scrape endpoints. With the current value (just
`text/plain`), it would fail to scrape.

In this commit the value is changed from `text/plain` to `text/plain;
version=0.0.4`.

See also [1] and [2]
[1] https://prometheus.io/docs/instrumenting/exposition_formats/
[2] https://prometheus.io/docs/prometheus/3.0/migration/