set proper metrics endpoint content type

[bogdan-rosianu]

Type

• Bug
• Feature
• Refactoring
• Performance improvement

Problem setting

• latest prometheus versions require strict content type of the metrics endpoint, displaying this error

received unsupported Content-Type \"text/html; charset=utf-8\" and no fallback_scrape_protocol specified for target

Proposed Changes

• set the endpoint content type from the prom-client library

How to test

• without this fix, the prometheus instance started with the project does not work properly, but it should work with this fix