When syncing, add target block via RPC

[amaury1093]

When syncing a node, the logs show a nice

⚙️  Syncing, target=#199957 (2 peers), best: #512 (0x5895…614e), finalized #512 (0x5895…614e), ⬇ 33.3kiB/s ⬆ 2.4kiB/s

However, I cannot find this target=#... number via RPC. In light-ui, where syncing can take some time, I'd like to show the target block.

I'm not 100% sure where to put it, maybe in system_health?

{
  peers: number,
  shouldHavePeers: boolean,
  isSyncing: boolean,
  // add this field:
  syncing: { startBlock: number, currentBlock: number, targetblock: number } | null
}

Note: I copied how eth did it, for inspiration: https://github.com/ethereum/wiki/wiki/JSON-RPC#eth_syncing

pinging maybe @tomusdrw @ascjones?

[tomaka]

That should rather be named something like bestNetworkBlock, so that it's more precisely defined, no?

[tomusdrw]

Yeah, I was thinking system_syncState - I don't think that health should need so many details.

[tomaka]

So Ethereum does this:

curl -X POST --data '{"jsonrpc":"2.0","method":"eth_syncing","params":[],"id":1}'

// Result
{
  "id":1,
  "jsonrpc": "2.0",
  "result": {
    startingBlock: '0x384',
    currentBlock: '0x386',
    highestBlock: '0x454'
  }
}

Any objection to doing exactly the same thing, except named system_syncState and with regular JavaScript numbers rather than hexadecimal strings?