Add volume metrics for CSI client

[xxx0624]

Summary

This CSI client encapsulates all things needed to communicate with the CSI driver daemon. As part of the client, this method GetVolumeMetrics can be used to get the given EBS volume stats. With this PR, we introduce a new struct to include more stats.

Implementation details

A new struct is added which include the Capacity and Used metrics for EBS volume.

Testing

New tests cover the changes: no

Run go mod vendor for both agent/ and ecs-agent/.

Description for the changelog

Update GetVolumeMetrics in the CSI client.

Licensing

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

[mye956]

Will these be used somewhere within the /agent directory in a future PR (or if it is already being used)? If so, we probably should also do a go mod vendor.

[xxx0624]

Will these be used somewhere within the /agent directory in a future PR (or if it is already being used)? If so, we probably should also do a go mod vendor.

Yes (not used for now). And I've run this for both agent/ and ecs-agent/