Metrics package

[lukasz-zimnoch]

Added metrics package which provides tools useful for gathering and exposing system metrics for external monitoring tools.

[Shadowfiend]

Is there an existing package used for similar stuff in any of our existing dependencies? Thinking specifically of libp2p.

[lukasz-zimnoch]

Is there an existing package used for similar stuff in any of our existing dependencies? Thinking specifically of libp2p.

Unfortunately, I didn't find anything fitting our needs.

[pdyraga]

There is a metrics package provided by go-ethereum: https://github.com/ethereum/go-ethereum/tree/master/metrics

Is it something we could not use for our needs? If so, can we summarize why for future-us?

If we did not explore this option, I think I am fine merging this PR and looking at go-ethereum metrics in the future.

[lukasz-zimnoch]

There is a metrics package provided by go-ethereum: https://github.com/ethereum/go-ethereum/tree/master/metrics

Is it something we could not use for our needs? If so, can we summarize why for future-us?

If we did not explore this option, I think I am fine merging this PR and looking at go-ethereum metrics in the future.

Yeah, this PR overlaps several things from go-ethereum, but also provides some useful functionality tailored for keep-* clients. Specifically, the observer logic allows us to define reusable metric observation loops in one place (on keep-core side) and avoid polluting the codebase with metric primitives which must be done in case we use bare go-ethereum metric package. Also, this PR is rather minimalistic because for now, we don't need all metric types and other magic. At last, we can always incorporate useful things from go-ethereum in the future if needed.