public OAuth2TokenDescriptorBuilder clientId(String clientId) {
        this.clientId = requireNonNull(clientId, "clientId");
        return this;
    }
{@code client_id} Token Introspection Response field,
OPTIONAL. Client identifier for the OAuth 2.0 client that requested this token.
oauth2/src/main/java/com/linecorp/armeria/common/auth/oauth2/OAuth2TokenDescriptorBuilder.java