Reward metric for validators

[RuslanGlaznyov]

Is your feature request related to a problem? Please describe.
I'm always frustrated when I want to count my rewards : )

Describe the solution you'd like
That would be great to get a reward count for the cycle for example. It can be from metric or by RPC

Additional context
I think that would good feature for validators to truck their rewards in dashboard

[damip]

Generalizing this within the context of something like the Node Manager plugin in massa station sounds like an interesting idea to be discussed.

FYI @SlnPons