fix for initialisation of the kubernetes provider

[gizas]

• Bug

What does this PR do?

The bug was identified here: #3346 (comment)
It addresses the correct initialisation of kubernetes provider when user in elastic-agent configures the
Sample of elastic-agent standalone configuraton manifest:

data:
  agent.yml: |-
    id: bea46550-8652-49e5-bfa2-7016cb60816c
    outputs:
      default:
        type: elasticsearch
        hosts:
          - 'https://elasticsearch:9200'
        username: '${ES_USERNAME}'
        password: '${ES_PASSWORD}'
    providers.kubernetes:
      node: ${NODE_NAME}
      add_resource_metadata:
        node: 
          enabled: false
        namespace:
          use_regex_include: true
          include_labels: ["pas"]

Why is it important?

It is important because this affects the correct initialisation of Kubernetes provider and finally affects the correct metadata enrichment of logs

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool
• I have added an integration test or an E2E test

How to test this PR locally

1. Clone code
2. Build elastic-agent image according to https://github.com/elastic/elastic-agent#testing-elastic-agent-on-kubernetes
3. Apply label in specific namespace: kubectl label namespace default pas2=pas
4. Apply standalone elastic-agent with:

providers.kubernetes:
      node: ${NODE_NAME}
      add_resource_metadata:
        node: 
          enabled: false
        namespace:
          use_regex_include: true
          include_labels: ["pas"]

5. Produce logs and verify that only namespace labels are present

Related issues

• Closes #[Kubernetes Provider] Allow wildcards or regex when specifying "include_labels" and "include_annotations" #3346

Screenshots

[mergify]

This pull request does not have a backport label. Could you fix it @gizas? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 8./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[gizas]

We noticed that the *config.Config pointer of https://github.com/elastic/elastic-agent/blob/main/internal/pkg/composable/providers/kubernetes/kubernetes.go#L49 is not initialised correctly based on the add_resource_metadata configuration that users provide in their manifest

The unpack method should had trigger the InitDefaults() according to PR but the initialisation does not work

[elastic-sonarqube]

Quality Gate failed

Failed conditions

0.0% Coverage on New Code (required ≥ 40%)

See analysis details on SonarQube

[gizas]

Closing as tests confirmed that it is not a bug!!!!!!
Everything is working correct in versions >= 8.12.0