public OAuth2TokenDescriptorBuilder extras(Map<String, String> extras) {
        this.extras.putAll(extras);
        return this;
    }
A {@link Map} of extra system-specific token parameters included with Token Introspection Response,
OPTIONAL.
oauth2/src/main/java/com/linecorp/armeria/common/auth/oauth2/OAuth2TokenDescriptorBuilder.java