[Feature] new prometheus metric : last synced beacon chain block number

[sophoah]

Problem/limitation At Hand

Beacon shard db block height is only available via a direct RPC call to the node, this prevent a proper monitoring via popular tool like grafana/prometheus.

Which component?

• [ ] consensus
• [ ] deploy script
• [ ] wallet
• [ ] explorer
• [ ] node script
• [ ] test automation
• [ ] demo app
• [X] others ...

Proposed Solution

Expose a new metric that highlight the node beacon shard DB last block synced

Alternatives Considered

in the meanwhile, external validator can use https://github.com/harmony-one/watchdog

Additional Context

Add any other context or screenshots about the feature request here.