[service] Allow running the Collector without any pipelines (#12613)

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

Adds a feature gate that allows starting the Collector without running
any pipelines. This will mainly be used for OpAMP, where the Collector
may need to be started with only extensions

Author: evan-bradley

.chloggen/allow-no-pipelines.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: Add `service.AllowNoPipelines` feature gate to allow starting the Collector without pipelines.
+
+# One or more tracking issues or pull requests related to the change
+issues: [12613]
+
+# (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: This can be used to start with only extensions.
+
+# 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: []

otelcol/config.go

@@ -9,6 +9,7 @@ import (
 
 	"go.opentelemetry.io/collector/component"
 	"go.opentelemetry.io/collector/service"
+	"go.opentelemetry.io/collector/service/pipelines"
 )
 
 var (
@@ -50,13 +51,13 @@ func (cfg *Config) Validate() error {
 
 	// Currently, there is no default receiver enabled.
 	// The configuration must specify at least one receiver to be valid.
-	if len(cfg.Receivers) == 0 {
+	if !pipelines.AllowNoPipelines.IsEnabled() && len(cfg.Receivers) == 0 {
 		return errMissingReceivers
 	}
 
 	// Currently, there is no default exporter enabled.
 	// The configuration must specify at least one exporter to be valid.
-	if len(cfg.Exporters) == 0 {
+	if !pipelines.AllowNoPipelines.IsEnabled() && len(cfg.Exporters) == 0 {
 		return errMissingExporters
 	}
 

otelcol/config_test.go

@@ -8,13 +8,14 @@ import (
 	"fmt"
 	"testing"
 
-	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 	config "go.opentelemetry.io/contrib/otelconf/v0.3.0"
 	"go.uber.org/zap/zapcore"
 
 	"go.opentelemetry.io/collector/component"
 	"go.opentelemetry.io/collector/config/configtelemetry"
 	"go.opentelemetry.io/collector/confmap/xconfmap"
+	"go.opentelemetry.io/collector/featuregate"
 	"go.opentelemetry.io/collector/pipeline"
 	"go.opentelemetry.io/collector/service"
 	"go.opentelemetry.io/collector/service/pipelines"
@@ -244,14 +245,31 @@ func TestConfigValidate(t *testing.T) {
 			cfg := tt.cfgFn()
 			err := xconfmap.Validate(cfg)
 			if tt.expected != nil {
-				assert.EqualError(t, err, tt.expected.Error())
+				require.EqualError(t, err, tt.expected.Error())
 			} else {
-				assert.NoError(t, err)
+				require.NoError(t, err)
 			}
 		})
 	}
 }
 
+func TestNoPipelinesFeatureGate(t *testing.T) {
+	cfg := generateConfig()
+	cfg.Receivers = nil
+	cfg.Exporters = nil
+	cfg.Service.Pipelines = pipelines.Config{}
+
+	require.Error(t, xconfmap.Validate(cfg))
+
+	gate := pipelines.AllowNoPipelines
+	require.NoError(t, featuregate.GlobalRegistry().Set(gate.ID(), true))
+	defer func() {
+		require.NoError(t, featuregate.GlobalRegistry().Set(gate.ID(), false))
+	}()
+
+	require.NoError(t, xconfmap.Validate(cfg))
+}
+
 func generateConfig() *Config {
 	return &Config{
 		Receivers: map[component.ID]component.Config{

service/pipelines/config.go

@@ -25,14 +25,20 @@ var (
 		featuregate.WithRegisterFromVersion("v0.112.0"),
 		featuregate.WithRegisterDescription("Controls whether profiles support can be enabled"),
 	)
+	AllowNoPipelines = featuregate.GlobalRegistry().MustRegister(
+		"service.AllowNoPipelines",
+		featuregate.StageAlpha,
+		featuregate.WithRegisterFromVersion("v0.122.0"),
+		featuregate.WithRegisterDescription("Allow starting the Collector without starting any pipelines."),
+	)
 )
 
 // Config defines the configurable settings for service telemetry.
 type Config map[pipeline.ID]*PipelineConfig
 
 func (cfg Config) Validate() error {
-	// Must have at least one pipeline.
-	if len(cfg) == 0 {
+	// Must have at least one pipeline unless explicitly disabled.
+	if !AllowNoPipelines.IsEnabled() && len(cfg) == 0 {
 		return errMissingServicePipelines
 	}
 

service/pipelines/config_test.go

@@ -120,6 +120,21 @@ func TestConfigValidate(t *testing.T) {
 	}
 }
 
+func TestNoPipelinesFeatureGate(t *testing.T) {
+	cfg := Config{}
+
+	require.Error(t, xconfmap.Validate(cfg))
+
+	gate := AllowNoPipelines
+	err := featuregate.GlobalRegistry().Set(gate.ID(), true)
+	require.NoError(t, err)
+	defer func() {
+		require.NoError(t, featuregate.GlobalRegistry().Set(gate.ID(), false))
+	}()
+
+	require.NoError(t, xconfmap.Validate(cfg))
+}
+
 func generateConfig(t *testing.T) Config {
 	t.Helper()