Include user context in a newly built Error

[aleksandrs-ledovskis]

What does this pull request do?

Captured errors now inherit the ElasticAPM::Context::User of the active transaction.

ErrorBuilder#add_current_transaction_fields already copies the transaction's labels and custom context onto each error, but silently omitted user - so errors reported during an authenticated request arrived with no user, even though the transaction (set via ElasticAPM.set_user) had one.

An explicitly-set error user takes precedence; otherwise the transaction's user is inherited. This aligns the Ruby agent with the Node.js' (1, 2), and Python's (1, 2) Elastic APM agents - all attaching the transaction's user to captured errors.

Why is it important?

Lacking a user's information the error metadata is less valuable. Cross-referencing with a specific set of circumstances is harder and more time consuming.

Checklist

• I have signed the Contributor License Agreement.
• My code follows the style guidelines of this project (See .rubocop.yml)
• I have rebased my changes on top of the latest main branch

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

• I have made corresponding changes to the documentation
• I have updated docs/release-notes/index.md
• I have updated docs/reference/supported-technologies.md
• Added an API method or config option? Document in which version this will be introduced

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