Content-Type for 'health/prometheus' metrics

[dmitriikazanin]

The problem is that since Prometheus 3.0 it's necessary to set Content-Type for responses with metrics. Otherwise, the following bug can occur:

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

At least it should be done for health/prometheus endpoint

mimetype="text/plain; version=0.0.4"

Updated release should be set for every dependent component.
Optional: it makes sense to set for all impacted service monitors 'fallbackScrapeProtocol: PrometheusText0.0.4' in order to avoid the mentioned bug in the future.