3542 iterable configuration option (helidon-io#3600)

* Added `Iterable` to the allowed types.

* Added Null check for argument type.

* Switched operands in null check.

* `allowedValues` method should just check for enum or return an empty list.

Author: dalexandrov

config/metadata-processor/src/main/java/io/helidon/config/metadata/processor/ConfigMetadataHandler.java

@@ -89,6 +89,7 @@ class ConfigMetadataHandler {
     private TypeMirror builderType;
     private TypeMirror configType;
     private TypeMirror erasedListType;
+    private TypeMirror erasedIterableType;
     private TypeMirror erasedSetType;
     private TypeMirror erasedMapType;
 
@@ -111,6 +112,7 @@ synchronized void init(ProcessingEnvironment processingEnv) {
         configType = elementUtils.getTypeElement("io.helidon.config.Config").asType();
         erasedListType = typeUtils.erasure(elementUtils.getTypeElement(List.class.getName()).asType());
         erasedSetType = typeUtils.erasure(elementUtils.getTypeElement(Set.class.getName()).asType());
+        erasedIterableType = typeUtils.erasure(elementUtils.getTypeElement(Iterable.class.getName()).asType());
         erasedMapType = typeUtils.erasure(elementUtils.getTypeElement(Map.class.getName()).asType());
     }
 
@@ -570,7 +572,8 @@ private OptionType type(ConfiguredOptionData annotation, ExecutableElement eleme
                 TypeMirror paramType = parameter.asType();
                 TypeMirror erasedType = typeUtils.erasure(paramType);
 
-                if (typeUtils.isSameType(erasedType, erasedListType) || typeUtils.isSameType(erasedType, erasedSetType)) {
+                if (typeUtils.isSameType(erasedType, erasedListType) || typeUtils.isSameType(erasedType, erasedSetType)
+                        || typeUtils.isSameType(erasedType, erasedIterableType)) {
                     DeclaredType type = (DeclaredType) paramType;
                     TypeMirror genericType = type.getTypeArguments().get(0);
                     return new OptionType(genericType.toString(), "LIST");
@@ -697,7 +700,7 @@ String toConfigKey(String methodName) {
     }
 
     static List<AllowedValue> allowedValues(Elements elementUtils, TypeElement typeElement) {
-        if (typeElement.getKind() == ElementKind.ENUM) {
+        if (typeElement != null && typeElement.getKind() == ElementKind.ENUM) {
             return typeElement.getEnclosedElements()
                     .stream()
                     .filter(element -> element.getKind().equals(ElementKind.ENUM_CONSTANT))

config/metadata/src/main/java/io/helidon/config/metadata/ConfiguredOption.java

@@ -104,7 +104,8 @@
     /**
      * Kind of this option.
      * Defaults to {@link Kind#VALUE},
-     * autodetects {@link Kind#LIST} if the parameter is an actual {@link java.util.List} or {@link java.util.Set}.
+     * autodetects {@link Kind#LIST} if the parameter is an actual {@link java.util.List}, {@link java.util.Set}
+     * or {@link java.lang.Iterable}.
      * {@link Kind#MAP} is detected as well, though the type must be a String to a primitive or string
      *
      * @return kind of configuration option