[CCIP-13058] docs: add metrics assessment

[makramkd]

Description

AI-generated assessment of beholder metrics coverage in this repo - intended to be reviewed by authors.

Context:

• CCIP is currently difficult to debug. We have embarked upon fixing this by first assessing the metrics that we do have across the stack - starting from the top (plugin) to the bottom (chain libraries).
• The goal is to create easily-executable runbooks and dashboards, for both agents and humans, in order to triage issues in production and to determine overall service health.

Related epic: https://smartcontract-it.atlassian.net/browse/CCIP-13065