fix(logging): export formatter-added attributes

[pmcollins]

Description

Some logging formatters, such as Celery’s, add fields to the passed-in LogRecord during formatting. OTel’s LoggingHandler currently collects attributes before calling the formatter, so formatter-added fields are omitted from exported logs.

This change collects attributes after formatting the record so that formatter-added fields are exported by OTel Python.

Fixes open-telemetry/opentelemetry-python#4446

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality not to work as expected)
• This change requires a documentation update

How Has This Been Tested?

• tox -e py314-test-instrumentation-logging — 66 tests passed
• Added test_formatter_added_attributes_are_exported, which verifies an attribute added by Formatter.format() is included in the exported log record
• Ran a local OTLP/HTTP integration test using Celery's TaskFormatter and verified that task_id and task_name are exported as structured log attributes
• Ran Ruff check and format validation on the changed source and test files

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated