build: track files list in synth.metadata
build: track files list in synth.metadata

Co-authored-by: Benjamin E. Coe <69440dae14aaa9fe2429b324bfce03449d09e1db@google.com>
Co-authored-by: Justin Beckwith <0b357449eb5b1ce3bad523b6be0681e856776d8e@gmail.com>

googleapis/nodejs-recaptcha-enterprise,googleapis/nodejs-recaptcha-enterprise,googleapis/nodejs-recaptcha-enterprise