Fix problems when enabling eks platform patch

[sikachu]

Description

This PR fixes a problem where users are unable to generate k8s manifests when they enable platform: 'eks' in their libsonnet configuration.

An attempt for fixing this has been made in #1286, but it tried to fix all the platforms at once including the documentation. However, in this PR, I would like to focus on just fixing the problem when enabling platform: 'eks' part.

Without this patch, user will get an error when they enable platform: 'eks' part like this:

local kp = (import 'kube-prometheus/main.libsonnet') + {
  values+:: {
    common+: {
      namespace: 'monitoring',
      platform: 'eks',
    },
  },
};

RUNTIME ERROR: Field does not exist: _config
	vendor/kube-prometheus/addons/aws-vpc-cni.libsonnet:80:17-37	
	Field "labels"	
	Field "metadata"	
	Field "resources/kubernetes-prometheusRuleAwsVpcCni"	
	During manifestation

To fix the problem, this patch passes $.values to make those values available for the platform patches. Then, in aws-vpc-cni.libsonnet, I changed those $.prometheus to use values from $.values or just hard-coded alternate values instead.

I think I will submit follow-up patch to update documentation where necessary to reflect the fact that you need to set platform: inside the common object, after this PR is merged.

By the way, this is the first time for me to work with Jsonnet and kube-prometheus codebase, so please feel free to give me feedback if I misunderstood anything. Thank you very much.

Type of change

• CHANGE (fix or feature that would cause existing functionality to not work as expected)
• FEATURE (non-breaking change which adds functionality)
• BUGFIX (non-breaking change which fixes an issue)
• ENHANCEMENT (non-breaking change which improves existing functionality)
• NONE (if none of the other choices apply. Example, tooling, build system, CI, docs, etc.)

Changelog entry

- Fixes a problem when enabling EKS platform patch.

[sikachu]

For this part, I couldn't find a good replacement for this as we no longer have access to $.prometheus in this scope. Please let me know if you have any suggestions on how this can be improved so we don't have to hard-coded these values here.

[paulfantom]

lgtm