I'm pretty sure that there is no simple way.
In fact, I'm not even sure this is a meaningful thing to measure. The request header and body are part of the HTTP application protocol. The certificate is sent as part of SSL / TLS setup ... before the HTTP stuff starts. And once SSL / TLS has started, the HTTP protocol is run "on top of" the SSL / TLS channel.
Even just measuring the amount of data in a HTTP request is tricky. A typical HTTP stack does not assemble the entire request message in one place, and does not keep a running total of the amount of data sent. Depending on the HTTP stack that you are using, you could (in theory) arrange to use a custom socket factory and socket streams that count the bytes sent.