telemetry(lambda): nit to use sessionDuration correctly for debug duration

[roger-zhangg]

Problem

This change has no customer facing impact.
duration -> sessionDuration (correct)
Previously this metrics is wrongly recorded to duration and got overwritten by the metrics wrapper

Solution

Change to use sessionDuration correctly

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[github-actions]

• This pull request modifies code in src/* but no tests were added/updated.
  • Confirm whether tests should be added or ensure the PR description explains why tests are not required.
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