statement-store: create metrics and dashboards

[alexggh]

The end goal is to be able to understand what is happening in the statement-store on production nodes, so we should extend the existing metrics with more to be able to answer questions like.

1. What is the total usage of statement-store.
2. Number of users/statements.
3. With how many peers is the node connected using the statement-store protocol.
4. How many statements were submitted.
5. How many statements were valid/invalid and reasons of being invalid.
6. Number of active subscriptions to the node.
7. Metrics regarding queues and workers, we should be able to quickly notice if workers gets stuck or overwhelmed and drop message.