Make k8sattributesprocessor available in Agent running in OTel mode

[ChrsMark]

What does this PR do?

This PR makes the k8sattributesprocessor available to users running Elastic Agent in otel mode.

Why is it important?

To allow users of Elastic Agent running as as OpenTelemetry Collector to configure pipelines containing the k8sattributesprocessor.

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. Prepare the binaryDEV=true EXTERNAL=true SNAPSHOT=true PLATFORMS=linux/amd64 PACKAGES=docker mage -v package
2. build the image cd build/package/elastic-agent/elastic-agent-linux-amd64.docker/docker-build && docker build -t agent-otel-dev .
3. Load to local cluster kind load docker-image agent-otel-dev:latest
4. prepare the ES secrets

kubectl create secret generic elastic-secret \
     --from-literal=es_endpoint='https://test-otel-blablabla.es.us-central1.gcp.cloud.es.io/' \                 
     --from-literal=es_password='asdfgp' \     
     --from-literal=es_user='elastic'

5. specify the image:

image: agent-otel-dev:latest
imagePullPolicy: Never

6. Deploy the manifest from https://github.com/elastic/opentelemetry-dev/pull/258 : k apply -f elastic-otel-collector.yaml
7. Verify the output:

2024-06-10T13:17:25.677Z	info	ResourceLog #0
Resource SchemaURL: 
Resource attributes:
     -> k8s.namespace.name: Str(kube-system)
     -> k8s.pod.name: Str(kindnet-6lgrl)
     -> k8s.container.restart_count: Str(0)
     -> k8s.pod.uid: Str(3844b30e-67ee-4bcc-98ac-8f2196852939)
     -> k8s.container.name: Str(kindnet-cni)
     -> k8s.daemonset.name: Str(kindnet)
     -> k8s.node.name: Str(kind-control-plane)
     -> k8s.pod.start_time: Str(2024-06-10T13:09:03Z)
ScopeLogs #0
ScopeLogs SchemaURL: 
InstrumentationScope  
LogRecord #0
ObservedTimestamp: 2024-06-10 13:17:25.420729149 +0000 UTC
Timestamp: 2024-06-10 13:17:25.300601954 +0000 UTC
SeverityText: 
SeverityNumber: Unspecified(0)
Body: Str(I0610 13:17:25.300466       1 main.go:223] Handling node with IPs: map[172.21.0.2:{}])
Attributes:
     -> logtag: Str(F)
     -> log.file.path: Str(/var/log/pods/kube-system_kindnet-6lgrl_3844b30e-67ee-4bcc-98ac-8f2196852939/kindnet-cni/0.log)
     -> log.iostream: Str(stderr)
     -> time: Str(2024-06-10T13:17:25.300601954Z)
Trace ID: 
Span ID: 
Flags: 0
LogRecord #1
ObservedTimestamp: 2024-06-10 13:17:25.420987445 +0000 UTC
Timestamp: 2024-06-10 13:17:25.300633177 +0000 UTC
SeverityText: 
SeverityNumber: Unspecified(0)
Body: Str(I0610 13:17:25.300494       1 main.go:227] handling current node)
Attributes:
     -> log.iostream: Str(stderr)
     -> logtag: Str(F)
     -> log.file.path: Str(/var/log/pods/kube-system_kindnet-6lgrl_3844b30e-67ee-4bcc-98ac-8f2196852939/kindnet-cni/0.log)
     -> time: Str(2024-06-10T13:17:25.300633177Z)
Trace ID: 
Span ID: 
Flags: 0
	{"kind": "exporter", "data_type": "logs", "name": "debug"}

[mergify]

This pull request does not have a backport label. Could you fix it @ChrsMark? 🙏
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)

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube

[michalpristas]

lgtm