Serve metrics for Prometheus

Closes #1618 (closed)

changed milestone to %Arkindex 1.5.2

requested review from @babadie

assigned to @vrigal

I tested with the Dockerfile build, but I did not managed with the binary build because of a license check error (even after upgrading the keygen policy): [ERROR] License check failed: No fingerprint generated.

added 8 commits

• 4079bbc2...018ecb1f - 4 commits from branch master
• 067158ac - Update builds
• 06835b98 - Serve metrics
• 7087d20f - Update README
• 5bc6c363 - Add a simple test case

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added 1 commit

• f946266d - Detect application port

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resolved all threads

added 6 commits

• 4899da1d - 1 commit from branch master
• 583e7f0c - Update builds
• 3b25f699 - Serve metrics
• 4512b31e - Update README
• 87594c33 - Add a simple test case
• beaad73b - Detect application port

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added 1 commit

• 0bbe9eb6 - Move middleware to the top

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resolved all threads

added 1 commit

• 0a702a44 - Simply use a view check

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resolved all threads

added 2 commits

• be3e6647 - Suggestions
• 3764a866 - Update tests

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added 10 commits

• 557a505d - 1 commit from branch master
• 79fe8e36 - Update builds
• 7aff19da - Serve metrics
• 931aa9d8 - Update README
• bd941fca - Add a simple test case
• a3f951d3 - Detect application port
• b5da35fe - Move middleware to the top
• 817355cd - Simply use a view check
• e5f529f6 - Suggestions
• 6df97eff - Update tests

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added 1 commit

• 53e2b1ab - Use text/plain + port from config

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I noticed that ./arkindex/manage.py gunicorn did not work, crashing on the int parse of None (default arg for metrics port).

I decided to simplify the logic:

• configure the port from YAML config, so it's more standard
• always have the metrics port set, default to 3000
• use the same setting in view & gunicorn
• no --metrics-port cli arg

I also set the response content type to text/plain as it's expected by the prometheus client (I did not tell you do to it though).

added 1 commit

• 41e1668e - Remove metrics port from dockerfiles

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merged