code: public com.google.protobuf.Duration getExpiration() {
      return expiration_ == null ? com.google.protobuf.Duration.getDefaultInstance() : expiration_;
    }
docs: <pre>
  Time a static file served by this handler should be cached
  by web proxies and browsers.
  </pre>
  
  <code>.google.protobuf.Duration expiration = 5;</code>