Implement metrics that tracks the base fee

[jp-imx]

Description

I couldn't find a metric in Prometheus to monitor gas price, so I added this one. If there's already something I can use I'm happy to adopt it and close this.

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Breaking changes

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Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually

Manual tests

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Documentation update

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Additional comments

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[Stefan-Ethernal]

Looks good, however, since this part of code is IBFT consensus protocol related, which is basically obsoleted, don't forget to register gauge here as well, which is related to polybft:
https://github.com/jp-imx/polygon-edge/blob/jp/add-basefee-gauge/consensus/polybft/consensus_metrics.go#L37

[goran-ethernal]

Hey @jp-imx . Thanks for the PR. You added this metric only for ibft.

To actually add it for polybft, please check out file consensus/polybft/consensus_metrics.go, function updateBlockMetrics. You can add the given metric there.

[jp-imx]

Thanks for pointing that out @Stefan-Ethernal and @goran-ethernal. I've now updated this

[Stefan-Ethernal]

LGTM

Cc @DannyS03 in order to update docs if necessary.