revert: remove test refactoring based on review feedback

Author: EdgeN8v

service/service.go

@@ -372,9 +372,7 @@ func Validate(ctx context.Context, set Settings, cfg Config) error {
 // Historically, attempting to register process metrics on unsupported platforms
 // (e.g. AIX) caused the Collector to fail at startup.
 // See https://github.com/open-telemetry/opentelemetry-collector/issues/12098
-//
-// It is defined as a variable to allow overriding in tests.
-var registerProcessMetrics = func(
+func registerProcessMetrics(
 	srv *Service,
 	goos string,
 	register func(component.TelemetrySettings, ...proctelemetry.RegisterOption) error,

service/service_test.go

@@ -882,22 +882,3 @@ func TestRegisterProcessMetrics_SupportedOS_RegisterFails_ReturnsError(t *testin
 	require.ErrorIs(t, err, wantErr)
 	require.Contains(t, err.Error(), "failed to register process metrics")
 }
-
-func TestNew_ProcessMetricsRegistrationFailure(t *testing.T) {
-	originalRegisterFunc := registerProcessMetrics
-	defer func() {
-		registerProcessMetrics = originalRegisterFunc
-	}()
-
-	registerProcessMetrics = func(srv *Service, goos string, register func(component.TelemetrySettings, ...proctelemetry.RegisterOption) error) error {
-		return errors.New("forced process metrics registration error")
-	}
-
-	set := newNopSettings()
-	cfg := newNopConfig()
-
-	_, err := New(context.Background(), set, cfg)
-
-	require.Error(t, err)
-	require.Contains(t, err.Error(), "forced process metrics registration error")
-}