New `metrics` block from slo-generator v1.3.0 makes TF 0.13 module fail

[ocervell]

Terraform struggles with complex nested variables validation. For instance, it cannot take a list of maps with arbitrary types for the map values: all values need to be either all required, or all of the same type. There is no concept of optional variables either.

See here and here for more context on those issues.

In order to avoid getting the issue with any addition in the SLO Config or Error Budget Policy, we should pass the SLO config file paths (with eventual variable replacement happening inside the module) instead of SLO config file contents, to bypass the variable validation (which slo-generator will take care of anyway).