Watchtower: Add prometheus metrics

[michaelarnauts]

Is your feature request related to a problem? Please describe.
I'm monitoring my containers trough cAdvisor with prometheus. This gives me nice metrics per container to see how they behave.

I'm also looking for a way to check if some containers are using an outdated image. I want to avoid running old software that might have security issues. Important is that I can't have my containers updated randomly during the day. This needs to be in an maintenance window.

I can already use --monitor-only, so this seems like a good idea. The only issue is that I now need a way to get this updatable information into prometheus.

Describe the solution you'd like
Provide metrics that prometheus can use to follow up when images are outdated. Usefull metrics might be:

• watchtower_last_checked{container="whatever"} timestamp
• watchtower_updatable{container="whatever"} 1/0
• watchtower_updated{container="whatever"} timestamp

[simskij]

Yeah, prometheus support seems like a really reasonable feature request for watchtower.
Feel free to create a pull request with the needed changes and I'll make sure to review it 🙏

[paulfantom]

I would propose a minor modification to @michaelarnauts request to implement metrics as follows:

Counter watchtower_updates_total{container="foo"} int
Gauge   watchtower_stale{container="foo"} 0/1
Gauge   watchtower_last_update{container="foo"} timestamp

As for last_checked I would say it isn't very important. Watchtower checks everything every X seconds, so this metric would be max X seconds behind last scrape, which doesn't give much new information.

Also I started implementing it :)

[simskij]

Awesome!

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[michaelarnauts]

@paulfantom do you have a merge request somewhere?

[simskij]

@paulfantom, may I assist you in any way in getting this feature ready for integration with master?

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[simskij]

Reopening this as I believe this is something we really should have.

[simskij]

I've started implementing some kind of embryo for this in #450