Metrics - update and implement missing metrics.

[ericsson49]

Current metrics (#178) implementation is partial, some metrics are missing.
Moreover, the metrics spec is to be reorganized (ethereum/beacon-metrics#9). So an update will be required.

[ericsson49]

Metrics are reorganized according to the spec.
Interop metrics are implemented, some others are missing and the list is unstable.
So, I think the issue is done for now and closing it.