[receiver/sapm] sapmreceiver adopts component.UseLocalHostAsDefaultHost feature gate (#30926)

**Description:**
sapmreceiver adopts `component.UseLocalHostAsDefaultHost` feature gate

**Link to tracking Issue:**
#30702 

**Documentation:**
Updated docs.

---------

Co-authored-by: Pablo Baeyens <[email protected]>

Author: atoulme

.chloggen/mx-psi_internal-localhostgate.yaml

@@ -27,6 +27,7 @@ subtext: |
     - receiver/influxdb
     - receiver/zookeeper
     - receiver/signalfx
+    - receiver/sapm
     - extension/jaegerremotesampling
     - receiver/jaeger
     - receiver/skywalking

receiver/sapmreceiver/README.md

@@ -26,6 +26,7 @@ The following settings are required:
 
 - `endpoint` (default = `0.0.0.0:7276`): Address and port that the SAPM
   receiver should bind to.
+  The `component.UseLocalHostAsDefaultHost` feature gate changes this to localhost:7276. This will become the default in a future release.
 
 The following setting are optional:
 

receiver/sapmreceiver/config_test.go

@@ -44,7 +44,7 @@ func TestLoadConfig(t *testing.T) {
 			id: component.NewIDWithName(metadata.Type, "tls"),
 			expected: &Config{
 				HTTPServerConfig: confighttp.HTTPServerConfig{
-					Endpoint: ":7276",
+					Endpoint: "0.0.0.0:7276",
 					TLSSetting: &configtls.TLSServerSetting{
 						TLSSetting: configtls.TLSSetting{
 							CertFile: "/test.crt",
@@ -58,7 +58,7 @@ func TestLoadConfig(t *testing.T) {
 			id: component.NewIDWithName(metadata.Type, "passthrough"),
 			expected: &Config{
 				HTTPServerConfig: confighttp.HTTPServerConfig{
-					Endpoint: ":7276",
+					Endpoint: "0.0.0.0:7276",
 				},
 				AccessTokenPassthroughConfig: splunk.AccessTokenPassthroughConfig{
 					AccessTokenPassthrough: true,

receiver/sapmreceiver/factory.go

@@ -16,12 +16,13 @@ import (
 	"go.opentelemetry.io/collector/consumer"
 	"go.opentelemetry.io/collector/receiver"
 
+	"github.com/open-telemetry/opentelemetry-collector-contrib/internal/common/localhostgate"
 	"github.com/open-telemetry/opentelemetry-collector-contrib/receiver/sapmreceiver/internal/metadata"
 )
 
 const (
-	// Default endpoints to bind to.
-	defaultEndpoint = ":7276"
+	// Default port to bind to.
+	defaultPort = 7276
 )
 
 // NewFactory creates a factory for SAPM receiver.
@@ -35,7 +36,7 @@ func NewFactory() receiver.Factory {
 func createDefaultConfig() component.Config {
 	return &Config{
 		HTTPServerConfig: confighttp.HTTPServerConfig{
-			Endpoint: defaultEndpoint,
+			Endpoint: localhostgate.EndpointForPort(defaultPort),
 		},
 	}
 }

receiver/sapmreceiver/trace_receiver_test.go

@@ -279,7 +279,7 @@ func TestReception(t *testing.T) {
 				// 1. Create the SAPM receiver aka "server"
 				config: &Config{
 					HTTPServerConfig: confighttp.HTTPServerConfig{
-						Endpoint: defaultEndpoint,
+						Endpoint: "0.0.0.0:7226",
 					},
 				},
 				sapm:        &splunksapm.PostSpansRequest{Batches: []*model.Batch{grpcFixture(now)}},
@@ -293,7 +293,7 @@ func TestReception(t *testing.T) {
 			args: args{
 				config: &Config{
 					HTTPServerConfig: confighttp.HTTPServerConfig{
-						Endpoint: defaultEndpoint,
+						Endpoint: "0.0.0.0:7226",
 					},
 				},
 				sapm:        &splunksapm.PostSpansRequest{Batches: []*model.Batch{grpcFixture(now)}},
@@ -382,7 +382,7 @@ func TestAccessTokenPassthrough(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			config := &Config{
 				HTTPServerConfig: confighttp.HTTPServerConfig{
-					Endpoint: defaultEndpoint,
+					Endpoint: "0.0.0.0:7226",
 				},
 				AccessTokenPassthroughConfig: splunk.AccessTokenPassthroughConfig{
 					AccessTokenPassthrough: tt.accessTokenPassthrough,