Change default metrics address to localhost:8888 (#11251)

Signed-off-by: Bogdan Drutu <bogdandrutu@gmail.com>

.chloggen/change-prom-address-to-localhost.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: 'breaking'
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: service/telemetry
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Change default metrics address to "localhost:8888" instead of ":8888"
+
+# One or more tracking issues or pull requests related to the change
+issues: [11251]
+
+# (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 behavior can be disabled by disabling the feature gate 'telemetry.UseLocalHostAsDefaultMetricsAddress'.
+
+# 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, api]

service/telemetry/config_test.go

@@ -71,7 +71,7 @@ func TestUnmarshalConfigDeprecatedAddressGateEnabled(t *testing.T) {
 	cfg := NewFactory().CreateDefaultConfig()
 	require.NoError(t, cm.Unmarshal(&cfg))
 	require.Len(t, cfg.(*Config).Metrics.Readers, 1)
-	assert.Equal(t, "", *cfg.(*Config).Metrics.Readers[0].Pull.Exporter.Prometheus.Host)
+	assert.Equal(t, "localhost", *cfg.(*Config).Metrics.Readers[0].Pull.Exporter.Prometheus.Host)
 	assert.Equal(t, 8888, *cfg.(*Config).Metrics.Readers[0].Pull.Exporter.Prometheus.Port)
 }
 

service/telemetry/factory.go

@@ -19,6 +19,13 @@ import (
 	"go.opentelemetry.io/collector/service/internal/resource"
 )
 
+var useLocalHostAsDefaultMetricsAddressFeatureGate = featuregate.GlobalRegistry().MustRegister(
+	"telemetry.UseLocalHostAsDefaultMetricsAddress",
+	featuregate.StageBeta,
+	featuregate.WithRegisterFromVersion("v0.111.0"),
+	featuregate.WithRegisterDescription("controls whether default Prometheus metrics server use localhost as the default host for their endpoints"),
+)
+
 // disableHighCardinalityMetricsFeatureGate is the feature gate that controls whether the collector should enable
 // potentially high cardinality metrics. The gate will be removed when the collector allows for view configuration.
 var disableHighCardinalityMetricsFeatureGate = featuregate.GlobalRegistry().MustRegister(
@@ -82,6 +89,11 @@ func NewFactory() Factory {
 }
 
 func createDefaultConfig() component.Config {
+	metricsHost := "localhost"
+	if !useLocalHostAsDefaultMetricsAddressFeatureGate.IsEnabled() {
+		metricsHost = ""
+	}
+
 	return &Config{
 		Logs: LogsConfig{
 			Level:       zapcore.InfoLevel,
@@ -103,7 +115,7 @@ func createDefaultConfig() component.Config {
 			Level: configtelemetry.LevelNormal,
 			Readers: []config.MetricReader{{
 				Pull: &config.PullMetricReader{Exporter: config.MetricExporter{Prometheus: &config.Prometheus{
-					Host: newPtr(""),
+					Host: &metricsHost,
 					Port: newPtr(8888),
 				}}}},
 			},

service/telemetry/factory_test.go

@@ -5,6 +5,7 @@ package telemetry
 
 import (
 	"context"
+	"strconv"
 	"testing"
 	"time"
 
@@ -15,8 +16,40 @@ import (
 	"go.uber.org/zap/zapcore"
 
 	"go.opentelemetry.io/collector/config/configtelemetry"
+	"go.opentelemetry.io/collector/featuregate"
 )
 
+func TestDefaultConfig(t *testing.T) {
+	tests := []struct {
+		expected string
+		gate     bool
+	}{
+		{
+			expected: "localhost",
+			gate:     true,
+		},
+		{
+			expected: "",
+			gate:     false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run("UseLocalHostAsDefaultMetricsAddress/"+strconv.FormatBool(tt.gate), func(t *testing.T) {
+			prev := useLocalHostAsDefaultMetricsAddressFeatureGate.IsEnabled()
+			require.NoError(t, featuregate.GlobalRegistry().Set(useLocalHostAsDefaultMetricsAddressFeatureGate.ID(), tt.gate))
+			defer func() {
+				// Restore previous value.
+				require.NoError(t, featuregate.GlobalRegistry().Set(useLocalHostAsDefaultMetricsAddressFeatureGate.ID(), prev))
+			}()
+			cfg := NewFactory().CreateDefaultConfig()
+			require.Len(t, cfg.(*Config).Metrics.Readers, 1)
+			assert.Equal(t, tt.expected, *cfg.(*Config).Metrics.Readers[0].Pull.Exporter.Prometheus.Host)
+			assert.Equal(t, 8888, *cfg.(*Config).Metrics.Readers[0].Pull.Exporter.Prometheus.Port)
+		})
+	}
+}
+
 func TestTelemetryConfiguration(t *testing.T) {
 	tests := []struct {
 		name    string