FR: metrics and stats

[kquinsland]

This issue is a request for two things that are related but might end up being implemented very differently.
If you want me to break this up into multiple issues, let me know!

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The first ask: basic operational metrics.

Currently, I am running this as a docker container and am using Google's ContainerAdvisor to expose basic metrics around CPU/Memory usage. This works well enough since my primary concern is "did I allocate enough RAM to this or should I add more?!". Since there are multiple processes running inside the container, there's no good way to get insight into the memory/cpu...etc of any one component. Not a concern for my purposes, but I can certainly imagine larger deployments where knowing cpu/disk/memory...etc consumption of node vs zoekt would come in very handy.

For these metrics, standard prometheus format would be ideal / most accessible for everybody.

The second ask: metrics about what's in the index.

• count of files in the index
• number of repos in the index
• how long indexing takes (combined with a counter for number of repos, I can calc average index time per repo...)
• success/fail count per repo (if - suddenly - the number of repos successfully refreshed/indexed inverts with the number of repos that could not be successfully refreshed/indexed, that's an alert that I need to go page somebody to go figure out what's happened to our git server!)