[pkg/otlp] Fix missing resource attributes default mapping when resource_attributes_as_tags: false

[KSerrania]

What does this PR do?

Backport of open-telemetry/opentelemetry-collector-contrib#6359.
Fixes the additional tags list not getting correctly set when resource_attributes_as_tags: false and instrumentation_library_metadata_as_tags: false.
Adds unit tests.

Motivation

Since #9472, when resource_attributes_as_tags: false and instrumentation_library_metadata_as_tags: false (default config), the default resource attributes mapping done by attributes.TagsFromAttributes wouldn't be added to the metric tags.

Describe how to test/QA your changes

Run the Agent with the OTLP pipeline, with the default settings. Check that the resource attributes that should be converted by default (eg. development.environment) are indeed converted into their corresponding tags.

Reviewer's Checklist

• If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• The appropriate team/.. label has been applied, if known.
• A release note has been added or the changelog/no-changelog label has been applied.
• Changed code has automated tests for its functionality.
• Adequate QA/testing plan information is provided if the qa/skip-qa label is not applied.
• If applicable, docs team has been notified or an issue has been opened on the documentation repo.
• If applicable, the need-change/operator and need-change/helm labels have been applied.
• If applicable, the config template has been updated.