Implement prometheus metrics

[minrk]

Proposed change

Drop deprecated statsd metrics in favor of prometheus, which we use everywhere. We don't use statsd anywhere anymore.

Alternative options

Continue not having prometheus metrics and/or don't drop statsd

Who would use this feature?

JupyterHub (or other) deployments that use prometheus, e.g. mybinder.org.

(Optional): Suggest a solution

• Add prometheus as a dependency
• Add metrics API endpoint
• Replace all existing statsd metrics with prometheus
• Add additional metrics, if any feel necessary (?)
• Remove statsd as a dependency
• CLI option to opt-in to (or -out of) metrics

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