Prometheus exporter for metrics

[aelkiss]

This adds a very simple plack application which we can run as a sidecar container for the ingest workers that will allow the metrics to be scraped.

To see it in action/test it locally:

docker compose up prometheus-exporter
curl http://localhost:9090 # should see the metric names
cp t/fixtures/simple/test/*.zip volumes_to_test # or put anything else in volumes_to_test
docker compose run validate
curl http://localhost:9090 # should see some values for some metrics

[aelkiss]

FYI @moseshll

[mwarin]

Looks great. I was able to variations on the following:

$ docker compose run --rm prometheus-exporter perl lib/HTFeed/JobMetrics.pm --operation add --metric ingest_handle_items --value 5000

...

$ curl http://localhost:9090 | grep ingest_handle_items
# TYPE ingest_handle_items counter
ingest_handle_items 5000

... and that's an APPROVE.

[mwarin]

I would expect to see use strict; and use warnings; at the top of every perl script, OR a comment with a compelling argument why not.