feat(observability): OpenTelemetry logs

[samugi]

Note for reviewers: commits are split and commented appropriately, I would recommend checking them one by one.

Summary

This PR adds OpenTelemetry formatted logs as a new
signal supported by the OpenTelemetry plugin.

It patches the ngx.log function (and adapts kong PDK log) to collect the logged lines and
enrich them with additional context info:

• severity
• introspection details
• timestamp

And for request-scoped logs also:

• Request ID
• Trace ID
• Span ID
• Trace flags

The log entries are protobuf-serialized and sent to the endpoint
configured in the OpenTelemetry plugin's config.logs_endpoint field.

Checklist

• The Pull Request has tests
• A changelog file has been added to CHANGELOG/unreleased/kong or adding skip-changelog label on PR if unnecessary. README.md
• The Pull Request has backports to all the versions it needs to cover
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - feat: OpenTelemetry logs docs.konghq.com#7583

Issue reference

KAG-4712

[ADD-SP]

And also I suggest not rebase commits when applying reviewing suggestions, we can use the following command for better view for the reviewer.

git commit --fixup <commit_id>

[samugi]

And also I suggest not rebase commits when applying reviewing suggestions, we can use the following command for better view for the reviewer.

git commit --fixup <commit_id>

yes absolutely. I wasn't planning to rebase (for now), which will make syncing (via cherry-pick) with the existing EE PR easier as well.

[ADD-SP]

LGTM, please rebase commits before the merge.

[team-gateway-bot]

Cherry-pick failed for master, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally.

git remote add upstream https://github.com/kong/kong-ee
git fetch upstream master
git worktree add -d .worktree/cherry-pick-13291-to-master-to-upstream upstream/master
cd .worktree/cherry-pick-13291-to-master-to-upstream
git checkout -b cherry-pick-13291-to-master-to-upstream
ancref=$(git merge-base 8f9b82d870985cdd5fb417d589ad2f7ddec967e4 c1f99a9ff2a5986dbd72e85d9897167df2124b77)
git cherry-pick -x $ancref..c1f99a9ff2a5986dbd72e85d9897167df2124b77