Add custom metrics config option

[Deezzir]

• Adds a new snap config option - dcgm-exporter-metrics-file.
• Adds a test for the new configuration.
• Divides tests into two classes: TestDCGMComponents and TestDCGMConfigs.

The user can set that config with the filename of the metrics file placed in /var/snap/dcgm/common prior. The dcgm-exporter wrapper will then check if the file exists in the directory above and supply it to the service.

The custom metrics file we developed will not be added alongside with the PR. It will be moved to the charm later when the feature is merged.

[gabrielcocenza]

It's looking good.

It's a personal preference but one of the highlights to me from pytest is that it's not necessary to create classes like when using unittest. I prefer not having classes, but no strong opinion

[aieri]

looks good, changes requested simply because I agree with Gabriel's comments

[jneo8]

One small suggestion

[aieri]

nice!

[gabrielcocenza]

Nice job!
Just some small details to polish