Setup integration tests for prometheus metrics

[Pokom]

Overview

There are currently no tests that check the validity of metrics that are exported. This means that effectively someone needs to run the application manually in order to validate that the different collectors are behaving appropriately. @skl mentioned that prometheus's golang client has a testutil package that can be used for this.

### Tasks
- [x] Setup tests for aws
- [x] Setup tests for gcp

[skl]

AWS tests introduced in:

• 36

[Pokom]

GCP tests intoduced in:

• 40