[confmap] Encode string-like map keys (open-telemetry#10137)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description

Our check for determining whether a map key can be encoded as a string
is too restrictive and doesn't into account types which are essentially
aliases for the string type and don't implement `UnmarshalText`.

I encountered this while trying to call
`(confmap.Conf).Marshal(*otelcol.Config)` when the config includes a
Prometheus receiver, which includes the [LabelName
type](https://github.com/prometheus/common/blob/main/model/labels.go#L98)
that does not implement an unmarshaling method. We can't guarantee that
all types will implement this, and [Go's print formatters
check](https://github.com/golang/go/blob/master/src/fmt/print.go#L803)
whether `(reflect.Value).Kind()` equals `reflect.String`, so I think
this will be an overall more robust approach.

<!--Describe what testing was performed and which tests were added.-->
#### Testing

I added two test cases to demonstrate when we will hit this case.

---------

Co-authored-by: Pablo Baeyens <pbaeyens31+github@gmail.com>

.chloggen/string-encoding.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: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: confmap
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Support string-like types as map keys when marshaling
+
+# One or more tracking issues or pull requests related to the change
+issues: [10137]
+
+# (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]

confmap/internal/mapstructure/encoder.go

@@ -168,11 +168,18 @@ func (e *Encoder) encodeMap(value reflect.Value) (any, error) {
 		if err != nil {
 			return nil, fmt.Errorf("error encoding key: %w", err)
 		}
-		key, ok := encoded.(string)
-		if !ok {
-			return nil, fmt.Errorf("%w key %q, kind: %v", errNonStringEncodedKey, iterator.Key().Interface(), iterator.Key().Kind())
+
+		v := reflect.ValueOf(encoded)
+		var key string
+
+		switch v.Kind() {
+		case reflect.String:
+			key = v.String()
+		default:
+			return nil, fmt.Errorf("%w, key: %q, kind: %v, type: %T", errNonStringEncodedKey, iterator.Key().Interface(), iterator.Key().Kind(), encoded)
 		}
-		if _, ok = result[key]; ok {
+
+		if _, ok := result[key]; ok {
 			return nil, fmt.Errorf("duplicate key %q while encoding", key)
 		}
 		if result[key], err = e.encode(iterator.Value()); err != nil {

confmap/internal/mapstructure/encoder_test.go

@@ -47,6 +47,8 @@ func (tID TestID) MarshalText() (text []byte, err error) {
 	return []byte(out), nil
 }
 
+type TestStringLike string
+
 func TestEncode(t *testing.T) {
 	enc := New(&EncoderConfig{
 		EncodeHook: TextMarshalerHookFunc(),
@@ -63,6 +65,22 @@ func TestEncode(t *testing.T) {
 			input: TestID("type"),
 			want:  "type_",
 		},
+		"MapWithTextMarshalerKey": {
+			input: map[TestID]TestSimpleStruct{
+				TestID("type"): {Value: "value"},
+			},
+			want: map[string]any{
+				"type_": map[string]any{"value": "value"},
+			},
+		},
+		"MapWithoutTextMarshalerKey": {
+			input: map[TestStringLike]TestSimpleStruct{
+				TestStringLike("key"): {Value: "value"},
+			},
+			want: map[string]any{
+				"key": map[string]any{"value": "value"},
+			},
+		},
 		"WithSlice": {
 			input: []TestID{
 				TestID("nop"),