[configopaque] configopaque.String implements fmt.Stringer (#9214)

**Description:**
configopaque.String implements `fmt.Stringer`, outputting [REDACTED]
when used as `fmt.Sprintf("%s", opaquestring)`

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

---------

Co-authored-by: Pablo Baeyens <pablo.baeyens@datadoghq.com>

.chloggen/configopaque_stringer.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: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+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)`
+
+# One or more tracking issues or pull requests related to the change
+issues: [9213]
+
+# (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:
+
+# 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: [api]

config/configopaque/opaque.go

@@ -5,11 +5,14 @@ package configopaque // import "go.opentelemetry.io/collector/config/configopaqu
 
 import (
 	"encoding"
+	"fmt"
 )
 
 // String alias that is marshaled in an opaque way.
 type String string
 
+var _ fmt.Stringer = String("")
+
 const maskedString = "[REDACTED]"
 
 var _ encoding.TextMarshaler = String("")
@@ -18,3 +21,7 @@ var _ encoding.TextMarshaler = String("")
 func (s String) MarshalText() ([]byte, error) {
 	return []byte(maskedString), nil
 }
+
+func (s String) String() string {
+	return maskedString
+}

config/configopaque/opaque_test.go

@@ -4,6 +4,7 @@
 package configopaque // import "go.opentelemetry.io/collector/config/configopaque"
 
 import (
+	"fmt"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -18,3 +19,14 @@ func TestStringMarshalText(t *testing.T) {
 		assert.Equal(t, "[REDACTED]", string(opaque))
 	}
 }
+
+func TestStringFmt(t *testing.T) {
+	examples := []String{"opaque", "s", "veryveryveryveryveryveryveryveryveryverylong"}
+	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)))
+	}
+}