[component] Print path to error in ValidateConfig (#12108)

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

Follow up to
#12102.

This prints the full path to where the error occurred in a config object
using available reflection metadata. Currently some paths will be
duplicated until we shift otelcol to call `component.ValidateConfig` on
`otelcol.Config` and remove manual calls to `Validate`, but no
information will be missing. If this is a concern we can do both steps
at once.

Example error messages:

```
collector server run finished with error: invalid configuration: receivers::otlp: must specify at least one protocol when using the OTLP receiver
service::telemetry: collector telemetry metrics reader should exist when metric level is not None, level is Normal
```

I'll clean this PR up once
#12102 is
merged, but you can see the latest commit for the changes relevant to
only this PR.

---------

Co-authored-by: Evan Bradley <[email protected]>
Co-authored-by: Pablo Baeyens <[email protected]>
Co-authored-by: Pablo Baeyens <[email protected]>

.chloggen/validateconfig-print-path.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: component
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Show path to invalid config in errors returned from `component.ValidateConfig`
+
+# One or more tracking issues or pull requests related to the change
+issues: [12108]
+
+# (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]

component/config.go

@@ -4,9 +4,11 @@
 package component // import "go.opentelemetry.io/collector/component"
 
 import (
+	"errors"
+	"fmt"
 	"reflect"
-
-	"go.uber.org/multierr"
+	"strconv"
+	"strings"
 )
 
 // Config defines the configuration for a component.Component.
@@ -31,46 +33,119 @@ type ConfigValidator interface {
 // ValidateConfig validates a config, by doing this:
 //   - Call Validate on the config itself if the config implements ConfigValidator.
 func ValidateConfig(cfg Config) error {
-	return validate(reflect.ValueOf(cfg))
+	var err error
+
+	for _, validationErr := range validate(reflect.ValueOf(cfg)) {
+		err = errors.Join(err, validationErr)
+	}
+
+	return err
+}
+
+type pathError struct {
+	err  error
+	path []string
+}
+
+func (pe pathError) Error() string {
+	if len(pe.path) > 0 {
+		var path string
+		sb := strings.Builder{}
+
+		_, _ = sb.WriteString(pe.path[len(pe.path)-1])
+		for i := len(pe.path) - 2; i >= 0; i-- {
+			_, _ = sb.WriteString("::")
+			_, _ = sb.WriteString(pe.path[i])
+		}
+		path = sb.String()
+
+		return fmt.Sprintf("%s: %s", path, pe.err)
+	}
+
+	return pe.err.Error()
+}
+
+func (pe pathError) Unwrap() error {
+	return pe.err
 }
 
-func validate(v reflect.Value) error {
+func validate(v reflect.Value) []pathError {
+	errs := []pathError{}
 	// Validate the value itself.
 	switch v.Kind() {
 	case reflect.Invalid:
 		return nil
 	case reflect.Ptr, reflect.Interface:
 		return validate(v.Elem())
 	case reflect.Struct:
-		var errs error
-		errs = multierr.Append(errs, callValidateIfPossible(v))
+		err := callValidateIfPossible(v)
+		if err != nil {
+			errs = append(errs, pathError{err: err})
+		}
+
 		// Reflect on the pointed data and check each of its fields.
 		for i := 0; i < v.NumField(); i++ {
 			if !v.Type().Field(i).IsExported() {
 				continue
 			}
-			errs = multierr.Append(errs, validate(v.Field(i)))
+			field := v.Type().Field(i)
+			path := fieldName(field)
+
+			subpathErrs := validate(v.Field(i))
+			for _, err := range subpathErrs {
+				errs = append(errs, pathError{
+					err:  err.err,
+					path: append(err.path, path),
+				})
+			}
 		}
 		return errs
 	case reflect.Slice, reflect.Array:
-		var errs error
-		errs = multierr.Append(errs, callValidateIfPossible(v))
+		err := callValidateIfPossible(v)
+		if err != nil {
+			errs = append(errs, pathError{err: err})
+		}
+
 		// Reflect on the pointed data and check each of its fields.
 		for i := 0; i < v.Len(); i++ {
-			errs = multierr.Append(errs, validate(v.Index(i)))
+			subPathErrs := validate(v.Index(i))
+
+			for _, err := range subPathErrs {
+				errs = append(errs, pathError{
+					err:  err.err,
+					path: append(err.path, strconv.Itoa(i)),
+				})
+			}
 		}
 		return errs
 	case reflect.Map:
-		var errs error
-		errs = multierr.Append(errs, callValidateIfPossible(v))
+		err := callValidateIfPossible(v)
+		if err != nil {
+			errs = append(errs, pathError{err: err})
+		}
+
 		iter := v.MapRange()
 		for iter.Next() {
-			errs = multierr.Append(errs, validate(iter.Key()))
-			errs = multierr.Append(errs, validate(iter.Value()))
+			keyErrs := validate(iter.Key())
+			valueErrs := validate(iter.Value())
+			key := stringifyMapKey(iter.Key())
+
+			for _, err := range keyErrs {
+				errs = append(errs, pathError{err: err.err, path: append(err.path, key)})
+			}
+
+			for _, err := range valueErrs {
+				errs = append(errs, pathError{err: err.err, path: append(err.path, key)})
+			}
 		}
 		return errs
 	default:
-		return callValidateIfPossible(v)
+		err := callValidateIfPossible(v)
+		if err != nil {
+			return []pathError{{err: err}}
+		}
+
+		return nil
 	}
 }
 
@@ -93,3 +168,39 @@ func callValidateIfPossible(v reflect.Value) error {
 
 	return nil
 }
+
+func fieldName(field reflect.StructField) string {
+	var fieldName string
+	if tag, ok := field.Tag.Lookup("mapstructure"); ok {
+		tags := strings.Split(tag, ",")
+		if len(tags) > 0 {
+			fieldName = tags[0]
+		}
+	}
+	// Even if the mapstructure tag exists, the field name may not
+	// be available, so set it if it is still blank.
+	if len(fieldName) == 0 {
+		fieldName = strings.ToLower(field.Name)
+	}
+
+	return fieldName
+}
+
+func stringifyMapKey(val reflect.Value) string {
+	var key string
+
+	if str, ok := val.Interface().(string); ok {
+		key = str
+	} else if stringer, ok := val.Interface().(fmt.Stringer); ok {
+		key = stringer.String()
+	} else {
+		switch val.Kind() {
+		case reflect.Ptr, reflect.Interface, reflect.Struct, reflect.Slice, reflect.Array, reflect.Map:
+			key = fmt.Sprintf("[%T key]", val.Interface())
+		default:
+			key = fmt.Sprintf("%v", val.Interface())
+		}
+	}
+
+	return key
+}