Relay balance metrics

[svyatonik]

part of #1290

There's couple of things that need to be solved:

• balance metric value must be nominated in chain tokens, not its minimal components. E.g. on Polkadot if relay has 100 DOTs on the balance, the metric must have value 100, not 100*10^10;
• I'll need to add metrics (and probably alerts later) to Rialto+Millau maintenance dashboard just to show that it works.

[svyatonik]