feat(opentelemetry): support variable resource attributes

[SuzyWangIBMer]

Summary

This change allows render resource attributes as lua code.
For example,

  config:
    traces_endpoint: "https://otlp-red-saas.instana.io:4318/v1/traces"
    resource_attributes:
      host.id: "$(headers.host)"

host.id is 0.0.0.0:9000 or depends on headers.host value.

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

Fix #12538

[samugi]

failing test is expected, let's rebase after #13877 is merged

[samugi]

Thank you so much for your contribution, this is great! I left a few comments for things that I feel could be improved a bit. There's some code duplication with request-transformer that we may want to take care of, but that refactor can be done later.

[samugi]

thanks for the quick action! I left a couple more suggestions, then I think it's good to go.

[samugi]

Looks good to me, thank you!!

Could you just update the documentation line for this configuration field in the plugin's schema? This is used to auto-generate docs.

[samugi]

great job and thanks again!

Edit: sorry about the failing CI, it's because the new schema description exceeded the length limit, if you undo that change I'll take care of updating the plugin's docs as required.