[cmd/mdatagen] Add go_struct.ignore_default flag to suppress default value generation for config fields (#15198)

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

This PR adds a `go_struct.ignore_default` flag to the mdatagen config
schema system. When set to true on a config field, mdatagen omits that
field from the generated `createDefaultConfig` function - emitting nil
for pointer fields and `configoptional.None` for optional fields —
instead of propagating defaults from the referenced schema.

<!-- Issue number if applicable -->
#### Link to tracking issue
Fixes #15156

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

1. Added and updated unit tests for default value parsing, JSON output,
resolver behavior, null/default formatting, optional object defaults,
and generated config helper behavior.
2. Exended `samplereceiver` example

Author: jkoronaAtCisco

.chloggen/mdatagen_null_default_values.yaml

@@ -0,0 +1,28 @@
+# 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. receiver/otlp)
+component: cmd/mdatagen
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Add `go_struct.ignore_default` flag to suppress default value generation for individual config fields.
+
+# One or more tracking issues or pull requests related to the change
+issues: [15156]
+
+# (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: |
+  Setting `go_struct.ignore_default: true` on a config field causes mdatagen to omit that field's default 
+  from the generated `createDefaultConfig` function, emitting `nil` for pointer fields 
+  and `configoptional.None` for optional fields.
+
+# 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]

cmd/mdatagen/internal/cfggen/generation.go

@@ -25,7 +25,7 @@ func NewCfgFns(rootPackage, componentPackage string) map[string]any {
 			}
 			imports, err := ExtractImports(cfg, rootPackage, componentPackage)
 			if err != nil {
-				return []string{}
+				panic(err)
 			}
 			return imports
 		},
@@ -202,7 +202,10 @@ func collectImports(md *ConfigMetadata, imports map[string]bool, rootPackage, co
 
 	if md.GoType != "" {
 		ref, err := ResolveGoTypeRef(md.GoType, rootPackage, componentPackage)
-		if err == nil && ref.ImportPath != "" {
+		if err != nil {
+			return fmt.Errorf("failed to resolve import for custom type %q: %w", md.GoType, err)
+		}
+		if ref.ImportPath != "" {
 			imports[ref.ImportPath] = true
 		}
 	}
@@ -217,11 +220,17 @@ func collectImports(md *ConfigMetadata, imports map[string]bool, rootPackage, co
 
 	if md.ResolvedFrom != "" {
 		ref, err := ResolveGoTypeRef(md.ResolvedFrom, rootPackage, componentPackage)
-		if err == nil && ref.ImportPath != "" {
+		if err != nil {
+			return fmt.Errorf("failed to resolve import for reference %q: %w", md.ResolvedFrom, err)
+		}
+		if ref.ImportPath != "" {
 			imports[ref.ImportPath] = true
 		}
 		refDesc := NewRef(md.ResolvedFrom)
 		if !refDesc.isInternal() {
+			if err := collectCustomDefaultImports(md, md.Default, imports, rootPackage, componentPackage); err != nil {
+				return err
+			}
 			return nil
 		}
 	}
@@ -267,6 +276,42 @@ func collectImports(md *ConfigMetadata, imports map[string]bool, rootPackage, co
 	return nil
 }
 
+func collectCustomDefaultImports(md *ConfigMetadata, defaultValue any, imports map[string]bool, rootPackage, componentPackage string) error {
+	if md == nil || md.GoStruct.IgnoreDefault {
+		return nil
+	}
+
+	switch typedValue := defaultValue.(type) {
+	case map[string]any:
+		if md.AdditionalProperties != nil {
+			return nil
+		}
+		for key, value := range typedValue {
+			prop := md.Properties[key]
+			if prop == nil {
+				continue
+			}
+			if err := collectImports(prop, imports, rootPackage, componentPackage); err != nil {
+				return err
+			}
+			if err := collectCustomDefaultImports(prop, value, imports, rootPackage, componentPackage); err != nil {
+				return err
+			}
+		}
+	case []any:
+		if md.Items == nil || md.Items.Type != "object" {
+			return nil
+		}
+		for _, item := range typedValue {
+			if err := collectCustomDefaultImports(md.Items, item, imports, rootPackage, componentPackage); err != nil {
+				return err
+			}
+		}
+	}
+
+	return nil
+}
+
 // FormatTypeName resolves a reference string to a Go type expression using GoTypeRef.
 func FormatTypeName(ref, rootPackage, componentPackage string) (string, error) {
 	tr, err := ResolveGoTypeRef(ref, rootPackage, componentPackage)
@@ -448,8 +493,10 @@ func generateValidatorName(propName string, desc *CustomValidatorConfig) string
 }
 
 func MapCustomDefaults(schema *ConfigMetadata, defaultValue any, rootPackage, componentPackage string) []string {
+	if schema.GoStruct.IgnoreDefault {
+		return nil
+	}
 	exps := make([]string, 0)
-
 	switch typedValue := defaultValue.(type) {
 	case map[string]any:
 		// is nested struct
@@ -483,12 +530,25 @@ func MapCustomDefaults(schema *ConfigMetadata, defaultValue any, rootPackage, co
 }
 
 func FormatDefaultValue(md *ConfigMetadata, name string, defaultValue any, rootPackage, componentPackage string) string {
+	if md.GoStruct.IgnoreDefault || (defaultValue == nil && !hasDefaultValue(md)) {
+		if md.IsPointer {
+			return "nil"
+		}
+		if md.IsOptional {
+			t, _ := resolveGoType(md, name, rootPackage, componentPackage)
+			return fmt.Sprintf("configoptional.None[%s]()", t)
+		}
+		return ""
+	}
 	exp := formatSimpleValue(md, name, defaultValue, rootPackage, componentPackage)
 	if md.IsPointer {
-		exp = "&" + exp
+		return "&" + exp
 	}
 	if md.IsOptional {
-		exp = fmt.Sprintf("configoptional.Some(%s)", exp)
+		if md.Type == "object" && md.Properties != nil {
+			return fmt.Sprintf("configoptional.Default(%s)", exp)
+		}
+		return fmt.Sprintf("configoptional.Some(%s)", exp)
 	}
 	return exp
 }
@@ -504,16 +564,19 @@ func FormatBaseValue(md *ConfigMetadata, name string, defaultValue any, rootPack
 func WrapDefaultValue(md *ConfigMetadata, varName string) string {
 	exp := varName
 	if md.IsPointer {
-		exp = "&" + exp
+		return "&" + exp
 	}
 	if md.IsOptional {
-		exp = fmt.Sprintf("configoptional.Some(%s)", exp)
+		if md.Type == "object" && md.Properties != nil {
+			return fmt.Sprintf("configoptional.Default(%s)", exp)
+		}
+		return fmt.Sprintf("configoptional.Some(%s)", exp)
 	}
 	return exp
 }
 
 func hasDefaultValue(md *ConfigMetadata) bool {
-	if md.Default != nil {
+	if !md.GoStruct.IgnoreDefault && md.Default != nil {
 		return true
 	}
 	for _, prop := range md.Properties {
@@ -559,8 +622,8 @@ func formatSimpleValue(md *ConfigMetadata, name string, defaultValue any, rootPa
 		return ""
 	}
 
-	// do not process further if "default" attribute not defined
-	if defaultValue == nil {
+	// do not process further if "ignore_default" attribute set
+	if md.GoStruct.IgnoreDefault {
 		return ""
 	}