Prometheus monitoring for GitLab Pages

This starts of the prometheus monitoring for GitLab Pages, and fixes #42 (closed)

Point to check:

• Are the metric names good, keeping Prometheus' conventions in mind?
• Golang, general style etc
• Shouldn't I do some voodoo magic to import this in the library?

TODO:

• Add gitlab_pages_last_update_seconds -- Seconds since 1970 to last update
• Add tests

/cc @nick.thomas @ayufan

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• f0f6b386 - Add packages with govendor add +e

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Example metrics:

# HELP gitlab_pages_domains_served_total The total number of sites served by this Pages app
# TYPE gitlab_pages_domains_served_total gauge
gitlab_pages_domains_served_total 7
# HELP gitlab_pages_domains_updated_total The total number of site updates processed since daemon start
# TYPE gitlab_pages_domains_updated_total counter
gitlab_pages_domains_updated_total 1
# HELP gitlab_pages_http_requests_total Total number of HTTP requests done serving
# TYPE gitlab_pages_http_requests_total counter
gitlab_pages_http_requests_total{code="302",method="GET"} 1
gitlab_pages_http_requests_total{code="404",method="GET"} 2
# HELP gitlab_pages_http_sessions_active The number of HTTP requests currently being processed
# TYPE gitlab_pages_http_sessions_active gauge
gitlab_pages_http_sessions_active 0
# HELP gitlab_pages_last_domain_update_seconds Seconds since Unix Epoc to the last update for all domains served
# TYPE gitlab_pages_last_domain_update_seconds counter
gitlab_pages_last_domain_update_seconds 1.485509286e+09

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• da4bdc68 - Prometheus monitoring for GitLab Pages

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mentioned in issue gitlab-com/infrastructure#1071 (closed)

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• 1e31fac9 - Prometheus monitoring for GitLab Pages

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• 85cab110 - Improve complexity

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• 6df60e27 - Improve complexity metrics

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assigned to @nick.thomas

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• 8a4f5c1e - Keep using golang v1.5.4 for now

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• 44cc9b13 - Improve complexity metrics

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• 76f8fdf8 - Fix dependencies

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• d4a4bab2 - Fix dependencies

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• 1c53d534 - Reduce the cyclomatic complexity of appMain()
• 45f8c120 - Revert changes to handling sockets in appMain

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• 19c147ec - Add Readme and remove unneeded log

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@zj with a few minor changes and a rebase / squash to get rid of the back-and-forth in the commits, I think this can be merged. Good work 👍

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• ddff37bc - Improve complexity metrics

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@nick.thomas Rebased, left with 2 commits. Let me know what you think

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LGTM 👍

mentioned in commit eae671b2

merged

@zj is this also configurable in Omnibus or not yet? Thanks :)

@axil Good catch. Not yet. @marin do you have some pointers for me to go on? Or could the build team pick this up?

@zj Do you have some pointers for me what too look at in this MR? Diff is 'uge.

Sorry about that, thats true. Didn't realise that. Anyway, in main.go I've declared a new flag to pass an address to the daemon to expose the metrics. Found here

Anyway, I found already how omnibus tries to handle these flags so I might just give this a whirl myself monday. :)

Great!

For cross linking: omnibus-gitlab!1300 (merged)