docs: Metrics, Logging and Prometheus

[illume]

Description

We're lacking documentation on setting up and using metrics/logging/prometheus.

Impact

It's not clear how metrics and logging works, what it can do for people, or if it's working as expected.

Environment and steps to reproduce

cd docs/
$ cat metrics.md
cat: metrics.md: No such file or directory

Expected behavior

Looking at metrics.md should show me how to set it up, how to use it, and tell me what information I can see in there.

Maybe we should have a separate doc section on logging and Prometheus?

Additional information

Information about metrics is in these files, and also in some closed PRs.

• backend/pkg/api/metrics_test.go
• backend/pkg/metrics/metrics.go
• https://github.com/kinvolk/nebraska/pulls?q=is%3Apr+metrics+is%3Aclosed

Some other open questions:

• is information from our home page also exported as a metric?
• what's important to alert on?
• is there any logging useful for general maintenance?