datadog-apm: deprecate opentelemetry datadog exporter

[mabdinur]

Description

Adds deprecation note to the Datadog Span Exporter example.

Type of change

Please delete options that are not relevant.

• Documentation change

How Has This Been Tested?

N/A

Does This PR Require a Contrib Repo Change?

Answer the following question based on these examples of changes that would require a Contrib Repo Change:

• The OTel specification has changed which prompted this PR to update the method interfaces of opentelemetry-api/ or opentelemetry-sdk/

• The method interfaces of test/util have changed

• Scripts in scripts/ that were copied over to the Contrib repo have changed

• Configuration files that were copied over to the Contrib repo have changed (when consistency between repositories is applicable) such as in

  • pyproject.toml
  • isort.cfg
  • .flake8

• When a new .github/CODEOWNER is added

• Major changes to project information, such as in:

  • README.md
  • CONTRIBUTING.md

• Yes. - Link to PR: datadog-apm: deprecate datadog exporter opentelemetry-python-contrib#900

• No.

Checklist:

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

[codeboten]

Pinging @majorgreys as the original author of the DD exporter

[codeboten]

As this is only a doc change, it doesn't need a line in the changelog since the package being deprecated lives in the contrib repo.