public Object descriptor() {
        return descriptor;
    }
Returns the descriptor of the type if and only if this type signature represents a descriptive type.
For reflection-based {@link DocServicePlugin}s, this will probably be a {@link Class}, but
other plugins may use an actual instance with descriptor information.
core/src/main/java/com/linecorp/armeria/server/docs/DescriptiveTypeSignature.java