Handle OpenMetrics prometheus metrics from applications

[lobkovilya]

What happened?

After #6783 prometheus can't scrape targets and returns the following error, so it was reverted. This issue covers this feature

[AyushSenapati]

expfmt.NegotiateIncludingOpenMetrics() only gets called when the hijacker fails to find a valid ContentType (CT) returned by the target applications. Upon debugging, it was found that Envoy returns Content-Type": "text/plain; charset=UTF-8" with out the version param. In result the responseFormat() is returning expfmt.FmtUnknown as we also validate the version param, which is why expfmt.NegotiateIncludingOpenMetrics() is called and it would return highest CT supported by the Prometheus Scraper (In our case this is expfmt.FmtOpenMetrics_1_0_0).

The actual metrics content of Envoy was text/plain but the Hijacker sets the CT header to Openmetrics, which is why prom scraper is failing to process.

I will update the hijacker to normalise the metrics content.

[jakubdyszkiewicz]

was it solved? Because I see