Add uperf throughput-delta-pct stats to detect UDP drops

[HughNhan]

Problem description:
In UPERF stream, the primary metric is the Gbps , the time-series of Tx byte counts. When UDP, viewing Gbps alone we do not see packet loss, and therefore may miss the picture.

timestamp_ms:1701713174720.9150 name:Txn3 nr_bytes:0 nr_ops:110
-------------------------------------------------------------------------------
timestamp_ms:1701713174720.9255 name:Total nr_bytes:919244111872 nr_ops:1795398830
Netstat statistics for this run
-------------------------------------------------------------------------------
Nic       opkts/s     ipkts/s      obits/s      ibits/s
eth0     13951267           0    60.27Gb/s     28.25b/s
-------------------------------------------------------------------------------
Run Statistics
Hostname            Time       Data   Throughput   Operations      Errors
-------------------------------------------------------------------------------
10.131.0.220     122.32s    92.84GB     6.52Gb/s    194709184        0.00
master           122.32s   856.11GB    60.12Gb/s   1795398848        0.00
-------------------------------------------------------------------------------
Difference(%)     -0.00%     89.16%       89.16%      89.16%       0.00%  

Solution:
Collect the Throughput difference metric, and index them.

[HughNhan]

Re-open the PR from the upstream repo branch instead of my fork.