A separate metric for actual network-transmitted bytes?

**Is your feature request related to a problem? Please describe.**

There is currently no way of telling how much data fluentbit transmits over the network to the log ingester without analyzing the ingester's http access logs or looking at the ingester's own metrics (if it has them at all).

Yes we have the `fluentbit_output_proc_bytes_total`, but it apparently counts the bytes before they are fed into the http client for transmission, and when the data is compressed before transmission (using gzip in my case), the actual amount transferred over the network becomes lower than that reported in `fluentbit_output_proc_bytes_total`.

**Describe the solution you'd like**

It would be nice to add a new metric to fluentbit such as `fluentbit_output_http_sent_bytes_total` that would measure actual bytes sent.

**Additional context**

Use case: traffic accounting for cost estimation and optimization in cloud-hosted environments.