Add ECMWF observability guidelines for logging and metrics

[sametd]

Context

This is a draft proposal to align observability guidelines across ECMWF software and services.
The objective of this PR is to collect broad feedback early, before we finalize requirements and structure.

Review approach

Please focus on whether the proposed direction is workable for your teams and platforms.
You are welcome to add colleagues as reviewers where you think it is useful.

Discussion scope for this PR

Please avoid deep implementation debates in this PR thread.
If needed, we can open follow-up issues/PRs for detailed technical discussions.

Next steps

After this review round:

1. Incorporate feedback into a revised version.
2. Add alerting guidance.
3. Add environment-specific collection guidance.
4. Add tracing guidance.