Hello M Varun
Welcome to Microsoft Q&A Platform, thanks for posting your query here.
Azure DevTest Labs is not listed as a separate service in the Cost Analysis blade.
However, as a work around you can still view the cost of your DevTest Labs resources.
In the Cost Management, select the Resource Group in the Group By, and then you can filter service with VM's. This way you can now see the cost of your DevTest Labs virtual machines.
Hope this helps.