docs: Add details on what metrics are being exposed in operator-sdk

[lilic]

See #719 (comment)

Documentation on what the actual metrics being exposed are.
Should we list out what exactly are the controller related metrics being recorded by the controller-runtime? And perhaps give an example of querying the endpoint to see these metrics in an example?
https://godoc.org/github.com/kubernetes-sigs/controller-runtime/pkg/internal/controller/metrics
https://github.com/kubernetes-sigs/controller-runtime/blob/master/pkg/internal/controller/metrics/metrics.go#L50-L58
Number of reconciliations
Number of reconciliation errors per controller
Reconciliation duration
Resource metrics like CPU, memory usage, file descriptor usage
Go runtime metrics like number of Go routines, GC duration

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[lilic]

/remove-lifecycle stale