feat: Implement Prometheus metric exporter

[scarmuega]

This PR introduces a /metrics endpoint that uses Prometheus format to expose metrics about the progress of the pipeline.

Each stage (source / sink) is responsible for notifying their progress as they process each event. This notifications are then aggregated via counters & gauges and exposed via HTTP using the well-known Prometheus encoding.

This PR includes the following metrics:

• chain_tip: the last detected tip of the chain (height)
• rollback_count: number of rollback events occurred
• source_current_slot: last slot processed by the source of the pipeline
• source_current_height: last height (block #) processed by the source of the pipeline
• source_event_count: number of events processed by the source of the pipeline
• sink_current_slot: last slot processed by the sink of the pipeline
• sink_event_count: number of events processed by the sink of the pipeline