add featuregate for k8s 1.28 native sidecar container

Signed-off-by: Benedikt Bongartz <bongartz@klimlive.de>

.chloggen/native_sidecar.yaml

@@ -0,0 +1,16 @@
+# 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. collector, target allocator, auto-instrumentation, opamp, github action)
+component: pkg/sidecar
+
+# A brief description of the change. Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Add native sidecar injection behind a feature gate which is disabled by default.
+
+# One or more tracking issues related to the change
+issues: [2376]
+
+# (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:

pkg/featuregate/featuregate.go

@@ -25,6 +25,19 @@ const (
 )
 
 var (
+	// EnableNativeSidecarContainers is the feature gate that controls whether a
+	// sidecar should be injected as a native sidecar or the classic way.
+	// Native sidecar containers have been available since kubernetes v1.28 in
+	// alpha and v1.29 in beta.
+	// It needs to be enabled with +featureGate=SidecarContainers.
+	// See:
+	// https://kubernetes.io/docs/reference/command-line-tools-reference/feature-gates/#feature-gates-for-alpha-or-beta-features
+	EnableNativeSidecarContainers = featuregate.GlobalRegistry().MustRegister(
+		"operator.sidecarcontainers.native",
+		featuregate.StageAlpha,
+		featuregate.WithRegisterDescription("controls whether the operator supports sidecar containers as init containers"),
+		featuregate.WithRegisterFromVersion("v0.98.0"),
+	)
 	EnableJavaAutoInstrumentationSupport = featuregate.GlobalRegistry().MustRegister(
 		"operator.autoinstrumentation.java",
 		featuregate.StageBeta,

pkg/sidecar/pod.go

@@ -25,6 +25,7 @@ import (
 	"github.com/open-telemetry/opentelemetry-operator/internal/config"
 	"github.com/open-telemetry/opentelemetry-operator/internal/manifests/collector"
 	"github.com/open-telemetry/opentelemetry-operator/internal/naming"
+	"github.com/open-telemetry/opentelemetry-operator/pkg/featuregate"
 )
 
 const (
@@ -47,7 +48,14 @@ func add(cfg config.Config, logger logr.Logger, otelcol v1beta1.OpenTelemetryCol
 		container.Env = append(container.Env, attributes...)
 	}
 	pod.Spec.InitContainers = append(pod.Spec.InitContainers, otelcol.Spec.InitContainers...)
-	pod.Spec.Containers = append(pod.Spec.Containers, container)
+
+	if featuregate.EnableNativeSidecarContainers.IsEnabled() {
+		policy := corev1.ContainerRestartPolicyAlways
+		container.RestartPolicy = &policy
+		pod.Spec.InitContainers = append(pod.Spec.InitContainers, container)
+	} else {
+		pod.Spec.Containers = append(pod.Spec.Containers, container)
+	}
 	pod.Spec.Volumes = append(pod.Spec.Volumes, otelcol.Spec.Volumes...)
 
 	if pod.Labels == nil {