[Filebeat] udp input - udp_read_buffer_length_gauge can be wrong

[andrewkroh]

The udp_read_buffer_length_gauge metric value comes directly from the value configured by the user in read_buffer.

However, the actual socket read buffer can differ when

• the user didn't configure a value (0 is reported and the OS uses sysctl net.core.rmem_default)
• the user's value was greater than sysctl net.core.rmem_max
• the user did configure a value and the OS allocated more space than requested for bookkeeping (see SO_RCVBUF, it normally doubles what you ask for)

My request is to populate this metric with the actual receive buffer size.

[elasticmachine]

Pinging @elastic/sec-deployment-and-devices (Team:Security-Deployment and Devices)