[configopaque] Implement fmt.GoStringer interface for String (#9271)

**Description:** Follow up to #9214.

To make sure the output when using `%#v` is redacted, we must also
implement the `fmt.GoStringer` interface.

**Link to tracking Issue:** Relates to #9213

**Testing:** Adds some tests with YAML and JSON as well as more thorough
testing of all `fmt` verbs

**Documentation:** Clarify that printing this is also redacted.

.chloggen/configopaque_stringer.yaml

@@ -7,7 +7,7 @@ change_type: breaking
 component: configopaque
 
 # A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
-note: configopaque.String implements `fmt.Stringer`, outputting [REDACTED] when used as `fmt.Sprintf("%s", opaquestring)`
+note: configopaque.String implements `fmt.Stringer` and `fmt.GoStringer`, outputting [REDACTED] when formatted with the %s, %q or %#v verbs`
 
 # One or more tracking issues or pull requests related to the change
 issues: [9213]

config/configopaque/opaque.go

@@ -8,11 +8,9 @@ import (
 	"fmt"
 )
 
-// String alias that is marshaled in an opaque way.
+// String alias that is marshaled and printed in an opaque way.
 type String string
 
-var _ fmt.Stringer = String("")
-
 const maskedString = "[REDACTED]"
 
 var _ encoding.TextMarshaler = String("")
@@ -22,6 +20,18 @@ func (s String) MarshalText() ([]byte, error) {
 	return []byte(maskedString), nil
 }
 
+var _ fmt.Stringer = String("")
+
+// String formats the string as `[REDACTED]`.
+// This is used for the %s and %q verbs.
 func (s String) String() string {
 	return maskedString
 }
+
+var _ fmt.GoStringer = String("")
+
+// GoString formats the string as `[REDACTED]`.
+// This is used for the %#v verb.
+func (s String) GoString() string {
+	return fmt.Sprintf("%#v", maskedString)
+}

config/configopaque/opaque_test.go

@@ -4,29 +4,79 @@
 package configopaque // import "go.opentelemetry.io/collector/config/configopaque"
 
 import (
+	"encoding/json"
 	"fmt"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"gopkg.in/yaml.v3"
 )
 
 func TestStringMarshalText(t *testing.T) {
 	examples := []String{"opaque", "s", "veryveryveryveryveryveryveryveryveryverylong"}
 	for _, example := range examples {
 		opaque, err := example.MarshalText()
 		require.NoError(t, err)
-		assert.Equal(t, "[REDACTED]", string(opaque))
+		assert.Equal(t, maskedString, string(opaque))
 	}
 }
 
+type TestStruct struct {
+	Opaque String `json:"opaque" yaml:"opaque"`
+	Plain  string `json:"plain" yaml:"plain"`
+}
+
+var example = TestStruct{
+	Opaque: "opaque",
+	Plain:  "plain",
+}
+
+func TestStringJSON(t *testing.T) {
+	bytes, err := json.Marshal(example)
+	require.NoError(t, err)
+	assert.Equal(t, `{"opaque":"[REDACTED]","plain":"plain"}`, string(bytes))
+}
+
+func TestStringYAML(t *testing.T) {
+	bytes, err := yaml.Marshal(example)
+	require.NoError(t, err)
+	assert.Equal(t, "opaque: '[REDACTED]'\nplain: plain\n", string(bytes))
+}
+
 func TestStringFmt(t *testing.T) {
 	examples := []String{"opaque", "s", "veryveryveryveryveryveryveryveryveryverylong"}
+	verbs := []string{"%s", "%q", "%v", "%#v", "%+v", "%x"}
 	for _, example := range examples {
-		// nolint S1025
-		assert.Equal(t, "[REDACTED]", fmt.Sprintf("%s", example))
-		// converting to a string allows to output as an unredacted string still:
-		// nolint S1025
-		assert.Equal(t, string(example), fmt.Sprintf("%s", string(example)))
+		for _, verb := range verbs {
+			t.Run(fmt.Sprintf("%s/%s", string(example), verb), func(t *testing.T) {
+				assert.Equal(t,
+					fmt.Sprintf(verb, maskedString),
+					fmt.Sprintf(verb, example),
+				)
+			})
+		}
+
+		for _, verb := range verbs {
+			t.Run(fmt.Sprintf("string(%s)/%s", string(example), verb), func(t *testing.T) {
+				// converting to a string allows to output as an unredacted string still:
+				var expected string
+				switch verb {
+				case "%s", "%v", "%+v":
+					expected = string(example)
+				case "%q", "%#v":
+					expected = "\"" + string(example) + "\""
+				case "%x":
+					expected = fmt.Sprintf("%x", []byte(example))
+				default:
+					t.Errorf("unexpected verb %q", verb)
+				}
+
+				assert.Equal(t,
+					expected,
+					fmt.Sprintf(verb, string(example)),
+				)
+			})
+		}
 	}
 }