Explain the concept of metaprogramming in Groovy and provide an example of how it can be used.

Metaprogramming in Groovy refers to the ability to modify and extend the behavior of the language itself or the behavior of existing classes at runtime. It allows developers to dynamically add, modify, or remove methods, properties, and other code artifacts during program execution. Metaprogramming in Groovy is made possible by its dynamic nature and powerful reflection capabilities.

One of the key features of metaprogramming in Groovy is the ability to handle method invocations that are not explicitly defined. This feature is known as "method missing" and allows developers to intercept method calls for which there is no corresponding method definition. By implementing the `methodMissing` method in a class or using the `@Mixin` annotation, developers can provide custom logic to handle these invocations. This enables the creation of domain-specific languages (DSLs) and the customization of behavior in a flexible and dynamic way.

Here's an example to illustrate how metaprogramming can be used in Groovy:

```
groovy`class Person {
String name
}

// Adding a method dynamically using metaprogramming
Person.metaClass.greet = {
"Hello, ${delegate.name}!"
}

def person = new Person(name: "John")
println person.greet()`
```
In this example, we define a `Person` class with a `name` property. Using metaprogramming, we dynamically add a `greet` method to the `Person` class at runtime. The `greet` method uses string interpolation to generate a greeting message. When we create an instance of `Person` and invoke the `greet` method, it dynamically executes the added method and prints "Hello, John!".

Another aspect of metaprogramming in Groovy is the ability to modify the structure and behavior of existing classes using AST (Abstract Syntax Tree) transformations. AST transformations allow developers to apply compile-time transformations to code, such as adding annotations, injecting code, or modifying class structures. This feature provides powerful customization capabilities and can be used for tasks like automatic method generation, aspect-oriented programming, or code generation.

Overall, metaprogramming in Groovy empowers developers to dynamically modify and extend the behavior of their applications. It provides a flexible and expressive way to customize code, create DSLs, and adapt applications to changing requirements. However, it should be used judiciously, as excessive metaprogramming can make code harder to understand and maintain.