Add missing successful restoration cardano db in prometheus aggregator

[jpraynaud]

Why

We have noticed that a metric is missing in the Prometheus endpoint of the aggregator: the metric recording the number of restoration of the Cardano database.

What

Implement the missing metric in the Prometheus endpoint of the aggregator.

How

• [x] Add cardano_db_total_restoration_since_startup counter metric
• [x] Record cardano_db_total_restoration_since_startup in the /statistics/snapshot route
• [x] Update Grafana dashboard