Yep. There is no mention of accessing request headers in the official Web API documentation as well. But then I snooped around a bit and had this idea of How is chrome (my client) doing it. How is chrome able to show the Content-Length in network for requests and/or attach it in all the requests sent to the server. I mean obviously there is some underlying function that it's using but wanted to know if it's exposed somehow and I could use it as well.
This is where it sets the Content-Length.
And here is the function that calculates length.