Measure HTTP request body length, number of events in body

[axw]

The server currently reports Content-Length in logs, and accumulates it in metrics, if it is not -1 -- i.e. not chunked encoding. We should also look into measuring the body size when chunked encoding is in use. Ideally we would also count the number of events in each payload.

This would be helpful in performance investigations, where the number of requests either remains stable or increases only slightly, but the payload size of requests increases significantly.

We will need to ensure the measurement does not negatively impact performance when the server's queue is full, and payloads are not fully decoded.