opentelemetry-sdk: fix serialization of objects in log handler

[xrmx]

Description

We should convert to string objects that are not AnyValues because otherwise exporter will fail later in the pipeline. While the export of all AnyValue types is not correct yet, exporter tests expects to being able to handle them and so they are already used in the handler.

Fixes #4509
Fixes #4409
Closes #4510
Closes #4514

Type of change

Please delete options that are not relevant.

• 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 to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• tox -e py310-test-opentelemetry-sdk

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

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

[lukaslihotzki-f]

This is a great improvement that fixes the existing issues, but invalid values (Sequence or Mapping containing invalid types) can still be put into the pipeline. Should the check be recursive? This could also be done in a follow-up PR. Fixing the simple case is more important in my opinion.

[xrmx]

This is a great improvement that fixes the existing issues, but invalid values (Sequence or Mapping containing invalid types) can still be put into the pipeline. Should the check be recursive? This could also be done in a follow-up PR. Fixing the simple case is more important in my opinion.

I hope this is enough, but we can revise it later if we need to handle more complex cases :)