[k8sattributesprocessor] Fall back to only container in pod in case n…

…o container name/id has been specified in the incoming resource attributes (#36844)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description

This PR adds a fallback logic to the processor by adding the container
attributes for an incoming resource without the container name/id being
explicitly set, provided the related pod only contains one container. If
there are more than one container in a pod, the container name/id still
have to be set explicitly.

<!-- Issue number (e.g. #1234) or full URL to issue, if applicable. -->
#### Link to tracking issue
Fixes #34189

<!--Describe what testing was performed and which tests were added.-->
#### Testing

Added unit tests

<!--Describe the documentation added.-->
#### Documentation

Adapted the readme to reflect the changes made in this PR

<!--Please delete paragraphs that you did not use before submitting.-->

---------

Signed-off-by: Florian Bacher <[email protected]>

.chloggen/k8sattributes-container-attributes.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: k8sattributesprocessor
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: For pods with only one container, the `container.id` and `k8s.container.name` are not longer required in the resource attributes to add the container attributes
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [34189]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

processor/k8sattributesprocessor/README.md

@@ -98,7 +98,9 @@ are then also available for the use within association rules. Available attribut
 Not all the attributes are guaranteed to be added. Only attribute names from `metadata` should be used for 
 pod_association's `resource_attribute`, because empty or non-existing values will be ignored.
 
-Additional container level attributes can be extracted provided that certain resource attributes are provided:
+Additional container level attributes can be extracted. If a pod contains more than one container,
+either the `container.id`, or the `k8s.container.name` attribute must be provided in the incoming resource attributes to
+correctly associate the matching container to the resource:
 
 1. If the `container.id` resource attribute is provided, the following additional attributes will be available:
    - k8s.container.name

processor/k8sattributesprocessor/processor.go

@@ -226,6 +226,15 @@ func (kp *kubernetesprocessor) addContainerAttributes(attrs pcommon.Map, pod *ku
 		if !ok {
 			return
 		}
+	// if there is only one container in the pod, we can fall back to that container
+	case len(pod.Containers.ByID) == 1:
+		for _, c := range pod.Containers.ByID {
+			containerSpec = c
+		}
+	case len(pod.Containers.ByName) == 1:
+		for _, c := range pod.Containers.ByName {
+			containerSpec = c
+		}
 	default:
 		return
 	}

processor/k8sattributesprocessor/processor_test.go

@@ -1264,6 +1264,80 @@ func TestProcessorAddContainerAttributes(t *testing.T) {
 				conventions.AttributeContainerImageName:       "test/app",
 			},
 		},
+		{
+			name: "fall back to only container",
+			op: func(kp *kubernetesprocessor) {
+				kp.podAssociations = []kube.Association{
+					{
+						Name: "k8s.pod.uid",
+						Sources: []kube.AssociationSource{
+							{
+								From: "resource_attribute",
+								Name: "k8s.pod.uid",
+							},
+						},
+					},
+				}
+				kp.kc.(*fakeClient).Pods[newPodIdentifier("resource_attribute", "k8s.pod.uid", "19f651bc-73e4-410f-b3e9-f0241679d3b8")] = &kube.Pod{
+					Containers: kube.PodContainers{
+						ByName: map[string]*kube.Container{
+							"app": {
+								Name:      "app",
+								ImageName: "test/app",
+								ImageTag:  "1.0.1",
+							},
+						},
+					},
+				}
+			},
+			resourceGens: []generateResourceFunc{
+				withPodUID("19f651bc-73e4-410f-b3e9-f0241679d3b8"),
+			},
+			wantAttrs: map[string]any{
+				conventions.AttributeK8SPodUID:          "19f651bc-73e4-410f-b3e9-f0241679d3b8",
+				conventions.AttributeK8SContainerName:   "app",
+				conventions.AttributeContainerImageName: "test/app",
+				conventions.AttributeContainerImageTag:  "1.0.1",
+			},
+		},
+		{
+			name: "multiple containers in the pod - do not fall back to any container",
+			op: func(kp *kubernetesprocessor) {
+				kp.podAssociations = []kube.Association{
+					{
+						Name: "k8s.pod.uid",
+						Sources: []kube.AssociationSource{
+							{
+								From: "resource_attribute",
+								Name: "k8s.pod.uid",
+							},
+						},
+					},
+				}
+				kp.kc.(*fakeClient).Pods[newPodIdentifier("resource_attribute", "k8s.pod.uid", "19f651bc-73e4-410f-b3e9-f0241679d3b8")] = &kube.Pod{
+					Containers: kube.PodContainers{
+						ByName: map[string]*kube.Container{
+							"app": {
+								Name:      "app",
+								ImageName: "test/app",
+								ImageTag:  "1.0.1",
+							},
+							"app2": {
+								Name:      "app2",
+								ImageName: "test/app",
+								ImageTag:  "1.0.1",
+							},
+						},
+					},
+				}
+			},
+			resourceGens: []generateResourceFunc{
+				withPodUID("19f651bc-73e4-410f-b3e9-f0241679d3b8"),
+			},
+			wantAttrs: map[string]any{
+				conventions.AttributeK8SPodUID: "19f651bc-73e4-410f-b3e9-f0241679d3b8",
+			},
+		},
 	}
 
 	for _, tt := range tests {