swarm: Create private storage and metrics API

[holisticode]

For swarm, it would be good to be able to collect stats and metrics on a node concerning storage (chunks, local DB, etc.) and chequebook properties (consumed and delivered services, service peers, balances, cheques, etc.).

The documentation for this new API will be maintained at: https://github.com/ethersphere/swarm/wiki/Storage-and-metrics-API

[karalabe]

Take a look at how we collect various system metrics currently via go-metrics in our code. I think you can fairly trivially add any metric you wish to collect to that subsystem.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[holisticode]

This is a high level ticket which encompasses several issues.

• A general swarm metrics system for network testing and observability has been implemented with ticket Implement metrics for swarm ethersphere/swarm#183, using the go-ethereum metrics code
• p2p accounting metrics have been implemented with PR Swap: accounting ethersphere/swarm#845 and Implement metrics persistance for p2p accounting ethersphere/swarm#1005
• There is an open ticket to document the p2p accounting metrics, which are user facing, here Document p2p accounting metrics ethersphere/swarm#1036

For these reasons it is best to close this high level ticket. For further needs about metrics new tickets can be created