fix: add block number value to the metrics

[kmd-fl]

Description

Add new fields last_seen_block and last_processed_block displaying the block number of the block from newHeader subscription handler.

Motivation

It will be helpful for alerts to detect chain de-synch

Proposed Changes

• add last_seen_block: Gauge
• add last_processed_block: Gauge
• use lower bits of the block number in metrics

Additional Notes

• We use i64 since u64 and hight aren't supported in prometheus due to some protobuf restriction
• It's fine to use only lower bits of the u256 block number since:
  1. most likely, we won't reach i64 soon
  2. these metrics will be used in alerts to detect the chain de-sync, lower bits should be enough for this task.