Hide profiles behind a feature gate (open-telemetry#11477)

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

This hides profiles support behind a feature gate, so folks have to
enable them explicitly to be able to use them.

.chloggen/profiles-featuregate.yaml

@@ -0,0 +1,25 @@
+# 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. otlpreceiver)
+component: service
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Hide profiles support behind a feature gate while it remains alpha.
+
+# One or more tracking issues or pull requests related to the change
+issues: [11477]
+
+# (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:
+
+# 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: [user]

service/pipelines/config.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 
 	"go.opentelemetry.io/collector/component"
+	"go.opentelemetry.io/collector/featuregate"
 	"go.opentelemetry.io/collector/pipeline"
 	"go.opentelemetry.io/collector/pipeline/pipelineprofiles"
 )
@@ -16,6 +17,14 @@ var (
 	errMissingServicePipelines         = errors.New("service must have at least one pipeline")
 	errMissingServicePipelineReceivers = errors.New("must have at least one receiver")
 	errMissingServicePipelineExporters = errors.New("must have at least one exporter")
+
+	serviceProfileSupportGateID = "service.profilesSupport"
+	serviceProfileSupportGate   = featuregate.GlobalRegistry().MustRegister(
+		serviceProfileSupportGateID,
+		featuregate.StageAlpha,
+		featuregate.WithRegisterFromVersion("v0.112.0"),
+		featuregate.WithRegisterDescription("Controls whether profiles support can be enabled"),
+	)
 )
 
 // Config defines the configurable settings for service telemetry.
@@ -31,8 +40,16 @@ func (cfg Config) Validate() error {
 	// only configured components.
 	for pipelineID, p := range cfg {
 		switch pipelineID.Signal() {
-		case pipeline.SignalTraces, pipeline.SignalMetrics, pipeline.SignalLogs, pipelineprofiles.SignalProfiles:
+		case pipeline.SignalTraces, pipeline.SignalMetrics, pipeline.SignalLogs:
 			// Continue
+		case pipelineprofiles.SignalProfiles:
+			if !serviceProfileSupportGate.IsEnabled() {
+				return fmt.Errorf(
+					"pipeline %q: profiling signal support is at alpha level, gated under the %q feature gate",
+					pipelineID.String(),
+					serviceProfileSupportGateID,
+				)
+			}
 		default:
 			return fmt.Errorf("pipeline %q: unknown signal %q", pipelineID.String(), pipelineID.Signal())
 		}

service/pipelines/config_test.go

@@ -9,15 +9,18 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 
 	"go.opentelemetry.io/collector/component"
+	"go.opentelemetry.io/collector/featuregate"
 	"go.opentelemetry.io/collector/pipeline"
+	"go.opentelemetry.io/collector/pipeline/pipelineprofiles"
 )
 
 func TestConfigValidate(t *testing.T) {
 	var testCases = []struct {
 		name     string // test case name (also file name containing config yaml)
-		cfgFn    func() Config
+		cfgFn    func(*testing.T) Config
 		expected error
 	}{
 		{
@@ -27,8 +30,8 @@ func TestConfigValidate(t *testing.T) {
 		},
 		{
 			name: "duplicate-processor-reference",
-			cfgFn: func() Config {
-				cfg := generateConfig()
+			cfgFn: func(*testing.T) Config {
+				cfg := generateConfig(t)
 				pipe := cfg[pipeline.NewID(pipeline.SignalTraces)]
 				pipe.Processors = append(pipe.Processors, pipe.Processors...)
 				return cfg
@@ -37,33 +40,33 @@ func TestConfigValidate(t *testing.T) {
 		},
 		{
 			name: "missing-pipeline-receivers",
-			cfgFn: func() Config {
-				cfg := generateConfig()
+			cfgFn: func(*testing.T) Config {
+				cfg := generateConfig(t)
 				cfg[pipeline.NewID(pipeline.SignalTraces)].Receivers = nil
 				return cfg
 			},
 			expected: fmt.Errorf(`pipeline "traces": %w`, errMissingServicePipelineReceivers),
 		},
 		{
 			name: "missing-pipeline-exporters",
-			cfgFn: func() Config {
-				cfg := generateConfig()
+			cfgFn: func(*testing.T) Config {
+				cfg := generateConfig(t)
 				cfg[pipeline.NewID(pipeline.SignalTraces)].Exporters = nil
 				return cfg
 			},
 			expected: fmt.Errorf(`pipeline "traces": %w`, errMissingServicePipelineExporters),
 		},
 		{
 			name: "missing-pipelines",
-			cfgFn: func() Config {
+			cfgFn: func(*testing.T) Config {
 				return nil
 			},
 			expected: errMissingServicePipelines,
 		},
 		{
 			name: "invalid-service-pipeline-type",
-			cfgFn: func() Config {
-				cfg := generateConfig()
+			cfgFn: func(*testing.T) Config {
+				cfg := generateConfig(t)
 				cfg[pipeline.MustNewID("wrongtype")] = &PipelineConfig{
 					Receivers:  []component.ID{component.MustNewID("nop")},
 					Processors: []component.ID{component.MustNewID("nop")},
@@ -73,17 +76,50 @@ func TestConfigValidate(t *testing.T) {
 			},
 			expected: errors.New(`pipeline "wrongtype": unknown signal "wrongtype"`),
 		},
+		{
+			name: "disabled-featuregate-profiles",
+			cfgFn: func(*testing.T) Config {
+				cfg := generateConfig(t)
+				cfg[pipeline.NewID(pipelineprofiles.SignalProfiles)] = &PipelineConfig{
+					Receivers:  []component.ID{component.MustNewID("nop")},
+					Processors: []component.ID{component.MustNewID("nop")},
+					Exporters:  []component.ID{component.MustNewID("nop")},
+				}
+				return cfg
+			},
+			expected: errors.New(`pipeline "profiles": profiling signal support is at alpha level, gated under the "service.profilesSupport" feature gate`),
+		},
+		{
+			name: "enabled-featuregate-profiles",
+			cfgFn: func(t *testing.T) Config {
+				require.NoError(t, featuregate.GlobalRegistry().Set(serviceProfileSupportGateID, true))
+
+				cfg := generateConfig(t)
+				cfg[pipeline.NewID(pipelineprofiles.SignalProfiles)] = &PipelineConfig{
+					Receivers:  []component.ID{component.MustNewID("nop")},
+					Processors: []component.ID{component.MustNewID("nop")},
+					Exporters:  []component.ID{component.MustNewID("nop")},
+				}
+				return cfg
+			},
+			expected: nil,
+		},
 	}
 
 	for _, tt := range testCases {
 		t.Run(tt.name, func(t *testing.T) {
-			cfg := tt.cfgFn()
+			cfg := tt.cfgFn(t)
 			assert.Equal(t, tt.expected, cfg.Validate())
+
+			// Clean up the profiles support gate, which may have been enabled in `cfgFn`.
+			require.NoError(t, featuregate.GlobalRegistry().Set(serviceProfileSupportGateID, false))
 		})
 	}
 }
 
-func generateConfig() Config {
+func generateConfig(t *testing.T) Config {
+	t.Helper()
+
 	return map[pipeline.ID]*PipelineConfig{
 		pipeline.NewID(pipeline.SignalTraces): {
 			Receivers:  []component.ID{component.MustNewID("nop")},