Extract Pebble's native metrics

[joshklop]

https://github.com/cockroachdb/pebble/blob/master/metrics.go

[Exca-DK]

Do you have some in mind that should be focused on?

[joshklop]

Ideally we can ingest them all into prometheus. A first PR that sets up the scaffolding could start with something simple like metrics.*Metrics.DiskSpaceUsage(). A few follow-ups could add everything on the *Metrics struct and FsyncLatencyBuckets.

May be useful: https://github.com/cockroachdb/pebble/blob/be1586403c9bdf0c6a97074f2762758a66f68806/db.go#L1905