add missing tracer for Dgraph datasource

[Umang01-hash]

Pull Request Template

Description:

• Closes issue #1234
• Add missing tracer for Dgraph datasource.

Checklist:

• [x] I have formatted my code using goimport and golangci-lint.
• [x] All new code is covered by unit tests.
• [x] This PR does not decrease the overall code coverage.
• [x] I have reviewed the code comments and documentation for clarity.

Thank you for your contribution!