fix(telemetry): prevent double counting of usage metrics

[rajib76]

Description

This is a fix for thr below bug: _[BUG] OpenTelemetry token/cost metrics double-counted in Langfuse due to duplicate reporting on parent and child spans #1267

Related Issues_

#1267

Documentation PR

Type of Change

Bug fix

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• [x] I ran hatch run prepare

Checklist

• [ ] I have read the CONTRIBUTING document
• [ ] I have added any necessary tests that prove my fix is effective or my feature works
• [ ] I have updated the documentation accordingly
• [ ] I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• [ ] My changes generate no new warnings
• [ ] Any dependent changes have been merged and published

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