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:1706563005688.6326 name:Total nr_bytes:43134222311 nr_ops:1316378
Netstat statistics for this run
-------------------------------------------------------------------------------
Nic       opkts/s     ipkts/s      obits/s      ibits/s
eth0        58059       15497    28.06Gb/s     8.18Mb/s
-------------------------------------------------------------------------------
Run Statistics
Hostname            Time       Data   Throughput   Operations      Errors
-------------------------------------------------------------------------------
10.130.2.129      12.31s    40.16GB    28.02Gb/s      1316099        0.00
master            12.31s    40.17GB    28.03Gb/s      1316380        0.00
-------------------------------------------------------------------------------
Difference(%)     -0.00%      0.02%        0.02%        0.02%       0.00%

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

Test
The content of the metric-data-0.csv

0,1706562995378,1706562996379,36.671877
0,1706562996380,1706562997380,36.135542
0,1706562997381,1706562998381,35.934417
0,1706562998382,1706562999382,36.336668
0,1706562999383,1706563000383,36.068500
0,1706563000384,1706563001384,35.666249
0,1706563001385,1706563002385,31.576698
0,1706563002386,1706563003386,32.314158
0,1706563003387,1706563004387,32.113033
0,1706563004388,1706563005588,26.597684
1,1706562995378,1706563005588,0.020000    <==== the new type, idx=1 with the correct start and end

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