Add internal localhostgate package (open-telemetry#30774)

**Description:** 

Follow up to open-telemetry/opentelemetry-collector/pull/8622

**Link to tracking Issue:** First step of open-telemetry#30702

.chloggen/mx-psi_internal-localhostgate.yaml

@@ -0,0 +1,29 @@
+# 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: all
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: "Add `component.UseLocalHostAsDefaultHost` feature gate that changes default endpoints from 0.0.0.0 to localhost"
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [30702]
+
+# (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 change affects the following components:
+    - TBF
+
+# 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: []

internal/common/localhostgate/featuregate.go

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+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+// package localhostgate defines a feature gate that controls whether server-like receivers and extensions use localhost as the default host for their endpoints.
+// This package is duplicated across core and contrib to avoid exposing the feature gate as part of the public API.
+// To do this we define a `registerOrLoad` helper and try to register the gate in both modules.
+// IMPORTANT NOTE: ANY CHANGES TO THIS PACKAGE MUST BE MIRRORED IN THE CORE COUNTERPART.
+// See https://github.com/open-telemetry/opentelemetry-collector/blob/main/internal/localhostgate/featuregate.go
+package localhostgate // import "github.com/open-telemetry/opentelemetry-collector-contrib/internal/common/localhostgate"
+
+import (
+	"errors"
+	"fmt"
+
+	"go.uber.org/zap"
+
+	"go.opentelemetry.io/collector/featuregate"
+)
+
+const UseLocalHostAsDefaultHostID = "component.UseLocalHostAsDefaultHost"
+
+// useLocalHostAsDefaultHostfeatureGate is the feature gate that controls whether
+// server-like receivers and extensions such as the OTLP receiver use localhost as the default host for their endpoints.
+var useLocalHostAsDefaultHostfeatureGate = mustRegisterOrLoad(
+	featuregate.GlobalRegistry(),
+	UseLocalHostAsDefaultHostID,
+	featuregate.StageAlpha,
+	featuregate.WithRegisterDescription("controls whether server-like receivers and extensions such as the OTLP receiver use localhost as the default host for their endpoints"),
+)
+
+// mustRegisterOrLoad tries to register the feature gate and loads it if it already exists.
+// It panics on any other error.
+func mustRegisterOrLoad(reg *featuregate.Registry, id string, stage featuregate.Stage, opts ...featuregate.RegisterOption) *featuregate.Gate {
+	gate, err := reg.Register(id, stage, opts...)
+
+	if errors.Is(err, featuregate.ErrAlreadyRegistered) {
+		// Gate is already registered; find it.
+		// Only a handful of feature gates are registered, so it's fine to iterate over all of them.
+		reg.VisitAll(func(g *featuregate.Gate) {
+			if g.ID() == id {
+				gate = g
+				return
+			}
+		})
+	} else if err != nil {
+		panic(err)
+	}
+
+	return gate
+}
+
+// EndpointForPort gets the endpoint for a given port using localhost or 0.0.0.0 depending on the feature gate.
+func EndpointForPort(port int) string {
+	host := "localhost"
+	if !useLocalHostAsDefaultHostfeatureGate.IsEnabled() {
+		host = "0.0.0.0"
+	}
+	return fmt.Sprintf("%s:%d", host, port)
+}
+
+// LogAboutUseLocalHostAsDefault logs about the upcoming change from 0.0.0.0 to localhost on server-like components.
+func LogAboutUseLocalHostAsDefault(logger *zap.Logger) {
+	if !useLocalHostAsDefaultHostfeatureGate.IsEnabled() {
+		logger.Warn(
+			"The default endpoints for all servers in components will change to use localhost instead of 0.0.0.0 in a future version. Use the feature gate to preview the new default.",
+			zap.String("feature gate ID", UseLocalHostAsDefaultHostID),
+		)
+	}
+}

internal/common/localhostgate/featuregate_test.go

@@ -0,0 +1,57 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package localhostgate
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"go.opentelemetry.io/collector/featuregate"
+)
+
+func setFeatureGateForTest(t testing.TB, gate *featuregate.Gate, enabled bool) func() {
+	originalValue := gate.IsEnabled()
+	require.NoError(t, featuregate.GlobalRegistry().Set(gate.ID(), enabled))
+	return func() {
+		require.NoError(t, featuregate.GlobalRegistry().Set(gate.ID(), originalValue))
+	}
+}
+
+func TestEndpointForPort(t *testing.T) {
+	tests := []struct {
+		port     int
+		enabled  bool
+		endpoint string
+	}{
+		{
+			port:     4317,
+			enabled:  false,
+			endpoint: "0.0.0.0:4317",
+		},
+		{
+			port:     4317,
+			enabled:  true,
+			endpoint: "localhost:4317",
+		},
+		{
+			port:     0,
+			enabled:  false,
+			endpoint: "0.0.0.0:0",
+		},
+		{
+			port:     0,
+			enabled:  true,
+			endpoint: "localhost:0",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.endpoint, func(t *testing.T) {
+			defer setFeatureGateForTest(t, useLocalHostAsDefaultHostfeatureGate, tt.enabled)()
+			assert.Equal(t, EndpointForPort(tt.port), tt.endpoint)
+		})
+	}
+}