Fix failing static checks

[danehlim]

Summary

Motivation:

1. This PR was raised: Update metrics interface to couple metric completion and publish #3803
2. Then this PR was raised (without the changes in the above PR taken into account): Add ACS attach resource responder to ecs-agent #3807
3. PR Update metrics interface to couple metric completion and publish #3803 was merged
4. PR Add ACS attach resource responder to ecs-agent #3807 was then subsequently successfully rebased and merged via Github, but now this line of code is not valid: https://github.com/aws/amazon-ecs-agent/blob/dev/ecs-agent/acs/session/attach_resource_responder.go#L72
5. Static checks are now failing

This PR addresses the line of code mentioned above to fix the failing static checks.

Implementation details

Update usage of metrics Entry interface Done method in attach resource responder.

Testing

Existing PR tests

Description for the changelog

Fix failing static checks

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.