Is there a Windows Performance Counter which contains the "Log Send Rate" for Availability Groups?
The DMV column log_send_rate will tell you Rate at which log records are being sent to the secondary databases, in kilobytes (KB)/second.
Depending on what SP + CU you are running, you may want to look into - KB 3012182 FIX: Log_Send_Rate column cannot reflect the rate accurately in SQL Server 2012 or in SQL Server 2014
Below PERFMON counter will tell you how many bytes were sent

SQL Server:Availability Replica --&gt; Bytes Sent to Replica/sec
Number of bytes sent to the remote availability replica per second. On the primary replica this is the number of bytes sent to the secondary replica. On the secondary replica this is the number of bytes sent to the primary replica.