Add block timeskew metric

[yacovm]

Why this should be merged

We have a metric that measures the time between building the block to accepting it. However it uses the block's timestamp. This value includes the time the VM builds a block as we need to compute the time and insert the time into the block before we build it.

To better compute the time between right before the block builder releases the block to the network and the block's acceptance it can be useful if we can estimate the time skew between the local time and the block's timestamp.

How this works

After the snowman engine builds a block, compute the difference between the current real time to the block's timestamp and publish it via a metrics gauge. Then we can use a monitor that divides this accumulated number (because it's a gauge that only adds) to the number of blocks built to get the average time skew.

How this was tested

I ran a modified local network and sent some Ethereum transactions to the C chain and then probed the metrics endpoint on the HTTP endpoint:

➜  ethcli git:(main) ✗ curl  -s "http://localhost:9650/ext/metrics" | grep skew | grep 'chain="C"'
avalanche_snowman_block_time_skew{chain="C"} 4.92657e+08
➜  ethcli git:(main) ✗ curl  -s "http://localhost:9660/ext/metrics" | grep skew | grep 'chain="C"'
avalanche_snowman_block_time_skew{chain="C"} 0
➜  ethcli git:(main) ✗ curl  -s "http://localhost:9670/ext/metrics" | grep skew | grep 'chain="C"'
avalanche_snowman_block_time_skew{chain="C"} 5.2301e+08

Need to be documented in RELEASES.md?

[Copilot]

Pull Request Overview

This PR introduces a new metric to record the time skew between the current time and the block’s timestamp. The key changes include:

• Adding a new prometheus gauge (blockTimeSkew) in the metrics struct.
• Initializing and registering the blockTimeSkew metric in newMetrics.
• Capturing and recording the time skew in the buildBlocks function.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
snow/engine/snowman/metrics.go	New gauge added and registered for block time skew.
snow/engine/snowman/engine.go	Captures and adds the block time skew to the gauge.

[StephenButtolph]

Should we include (either in the name or the help text) that this is specifically for blocks built by the node? Perhaps making the name blks_built_time_skew to align more the with the existing blks_built metric (which is used to actually get the time skew per block)

[yacovm]

Changed the name

[yacovm]

and changed the description too