Refactoring after #1481 - removing not needed env variables

[pszkamruk-splunk]

Description:
Cleanup and refactor work to prevent code duplication related to setting Kubernetes attributes by using k8sAttributesProcessor for cluster receiver introduced by PR

Once following two PRs will be merged we can merge this one:

• Contrib repo PR
• Chart repo PR

We need also decide what to do with resource/add_collector_k8s: link
as this is using env variables which we want to remove, but the functionality of adding additional parameters is covered by k8sattributes/metrics, so probably it is safe to remove resource/add_collector_k8s from configuration

[jvoravong]

For resource/add_collector_k8s:

• Replace this with a k8sattributes processor.
• Aim to use a single instance of the k8sattributes processor if possible. If required, add a second instance (e.g., k8sattributes/add_collector) for handling collector metrics specifically.

Functional Tests cover internal collector metrics and attributes (e.g., otelcol_exporter_send_failed_spans).

• Check to see if you can get the functional tests to pass with using only a single k8sattributes processor instance. Add a second processor instance (named something like k8sattributes/add_collector) because the tests failed and it is necessary.