CloudWatch Agent Config Doesn't Support `append_dimensions` for `nvidia_gpu` Metrics

**Is your feature request related to a problem? Please describe.**
I'm trying to set up CloudWatch agent to monitoring GPU usage on a group of ECS clusters. I want to append an ECS cluster name dimension to the GPU metrics. This is supported for all other metrics according to the [doc](https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/CloudWatch-Agent-Configuration-File-Details.html#CloudWatch-Agent-Configuration-File-Metricssection). However, GPU metric seems to be the only one I can't append custom dimensions to. Is there a specific reason why GPU metric is so special?

**Describe the solution you'd like**
I want to use `append_dimensions` in the `nvidia_gpu` section.

**Describe alternatives you've considered**
Didn't come up with an alternative.

**Additional context**
N/A

Start by locating the configuration handling for the `nvidia_gpu` section and compare it with metric sections that already support `append_dimensions`. Done means a configuration using `append_dimensions` in `nvidia_gpu` is accepted and the resulting GPU metrics include the requested ECS cluster dimension.

Written by the indexing model from the issue text.