Shomei - metric for chain head (like ethereum_blockchain_height)

[diana-borbe-consensys]

Description

As an Linea Operator, I want to be able to easily monitor shomei sync status.

Acceptance Criteria

• Explose shomei metric to follow head.
• This metric will be similar to besu metric ethereum_blockchain_height

Steps to Reproduce (Bug)

1. [Step 1]
2. [Step 2]
3. [Step ...]

Expected behavior: [What you expect to happen]

Actual behavior: [What actually happens]

Frequency: [What percentage of the time does it occur?]

Logs (if a bug)

Please post relevant logs from Besu (and the consensus client, if running proof of stake) from before and after the issue.

Versions (Add all that apply)

• Software version: [besu --version]
• Java version: [java -version]
• OS Name & Version: [cat /etc/*release]
• Kernel Version: [uname -a]
• Virtual Machine software & version: [vmware -v]
• Docker Version: [docker version]
• Cloud VM, type, size: [Amazon Web Services I3-large]
• Consensus Client & Version if using Proof of Stake: [e.g. Teku, Lighthouse, Prysm, Nimbus, Lodestar]

Smart contract information (If you're reporting an issue arising from deploying or calling a smart contract, please supply related information)

• Solidity version [solc --version]
• Repo with minimal set of deployable/reproducible contract code - please provide a link
• Please include specifics on how you are deploying/calling the contract
• Have you reproduced the issue on other eth clients

Additional Information (Add any of the following or anything else that may be relevant)

• Besu setup info - genesis file, config options
• System info - memory, CPU