Report request body buffered to disk

About `client_body_buffer_size` NGINX documentation says:

> Sets buffer size for reading client request body. In case the request body is larger than the buffer, the whole body or only its part is written to a temporary file. By default, buffer size is equal to two memory pages. This is 8K on x86, other 32-bit platforms, and x86-64. It is usually 16K on other 64-bit platforms.

We want to be able to know when this occurs by adding a field to the reported request that tracks this particular behavior.