use GunicornInternalPrometheusMetrics

[charlienegri]

Summary: this avoids starting an extra http server for the metrics (which is what GunicornPrometheusMetrics does) at port 9200,
the problem with that is that when running the container locally all metrics get redirected at the right endpoint via port 9200, but when running in kubernetes the custom metrics and the flask standard metrics seem to be sent to different ports... here we try to avoid that
it will need an edit to the tjeneste repo to scrape port 8000 instead of 9200

Related issue: #220

Suggested reviewer(s):

Reviewer checklist:

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[charlienegri]

if this works, then https://gitlab.met.no/tjenester/s-enda/-/merge_requests/2929

[charlienegri]

local test works... but then again this is no guarantee