Protocol usage metrics/statistics

[jpraynaud]

Why

We want to provide consistent usage metrics and statistics for Mithril networks. In particular, the information should be accessible in the Mithril Protocol Insights dashboard. These information need to be implemented at the aggregator level and should be meaningful in a decentralized setup.

What

Record protocol usage metrics and provide statistics for them.

How

• Refactor Prometheus metrics for signer #1990
• Expose Prometheus metrics for aggregator #1980
• Create Grafana dashboard for aggregator #2001
• Record aggregator metrics in event store #2023
• Implement the metrics in the Mithril Protocol Insights dashboard