Refactor Prometheus metrics for signer

[jpraynaud]

Why

We want to simplify the code of the metrics module in the signer: avoid repeating code for declaration of the metrics, provide a unique entry point for updating a metric and make the server available for other nodes.

What

Refactor the metrics module in the signer.

How

• Create a macro to generate the service with described metrics
• Create a MetricCollector on which we can update or fetch the metric
• Create a new internal/mithril-metrics crate:
  • Make some tooling for declaration of new metrics
  • Migrate the metrics server (which will use a MetricsServiceExporter trait as a dependency)