[Kubernetes Provider] Fix namespace filter on pod and service watchers

[constanca-m]

What does this PR do?

The watchers now take into account the namespace specified by the user.

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. Create setup:

> kind create cluster
> elastic-package stack up -d -v --version=8.15.0-SNAPSHOT
> docker network connect elastic-package-stack_default kind-control-plane

2. Build elastic-agent:

> EXTERNAL=true DEV=true PLATFORMS=linux/amd64 SNAPSHOT=TRUE PACKAGES=docker mage package

3. Build docker image:

> cd build/package/elastic-agent/elastic-agent-linux-amd64.docker/docker-build
> docker build -t custom-agent-image .

4. Load this image in your kind cluster:

> kind load docker-image custom-agent-image:latest

5. Deploy elastic agent managed with that image, associate it to a policy with Kubernetes integration with pod data stream enabled. Filter by namespace.

Related issues

Relates elastic/beats#38978.

[mergify]

This pull request does not have a backport label. Could you fix it @constanca-m? 🙏
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.

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[constanca-m]

/test

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
69.2% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube

[michalpristas]

should this be backported to 8.14.2?

[constanca-m]

should this be backported to 8.14.2?

The fix is only applied on beats for 8.15 (PR here) @michalpristas, so I am trying to fix it in the same version everywhere @michalpristas

[blakerouse]

Looks good.

We really need to get integration testing for kubernetes.

[michalpristas]

@blakerouse you know if we have some backing issue for adding it?

[pierrehilbert]

@michalpristas #4984 would be a first good progress here!