feat: add block time metric

[Thegaram]

1. Purpose or design rationale of this PR

Track block time, defined as the gap between the timestamps of two consecutive blocks.

Note: This is not exactly the same as the time it takes to produce a block but it's correlated. The relevant code is here:

go-ethereum/consensus/clique/clique.go

Lines 557 to 560 in 62926d4

	header.Time = parent.Time + c.config.Period
	if header.Time < uint64(time.Now().Unix()) {
	header.Time = uint64(time.Now().Unix())
	}

E.g. if block #100 is at 100s, then we set block #101's timestamp as 103s, but in reality we seal and broadcast this block at 106s. Then, block #102's timestamp will be set as 106 or higher. So this metric will capture the delay.

I decided to track this metric in writeBlockWithState because this way it's tracked both in signer nodes (from WriteBlockWithState, called by worker) and follower nodes (from InsertChain).

2. PR title

Your PR title must follow conventional commits (as we are doing squash merge for each PR), so it must start with one of the following types:

• feat: A new feature

3. Deployment tag versioning

Has the version in params/version.go been updated?

• This PR doesn't involve a new deployment, git tag, docker image tag, and it doesn't affect traces
• Yes

4. Breaking change label

Does this PR have the breaking-change label?

• This PR is not a breaking change
• Yes