sanjeet006py opened a new pull request, #7132:
URL: https://github.com/apache/hbase/pull/7132

   Cherry-pick of https://github.com/apache/hbase/pull/6868 for HBase 2. 
   
   Modifications done for HBase 2:
    - Sync HBase client read path is different in HBase 2 from HBase 3 and 4. 
Thus, modifications has been done to handle only this part.
    - In HBase 3 and 4, scan metrics by region are available for sync and async 
client both for STRONG consistency and TIMELINE consistency. But in HBase 2, 
for sync client and TIMELINE consistency, scan metrics by region is not 
supported. If users enable the flag then in constructor of `ClientScanner` we 
disable it and log a warning stating the same. 
    - In `ClientScanner` used by sync clients, two modifications have been done 
to add support for scan metrics by region:
           - In `moveToNextRegion()` we conditionally call 
`scanMetrics.moveToNextRegion()`.
           - We initialize the `ScanMetricsRegionInfo` at same time when we 
capture current region being scanned in `ClientScanner`. Code to capture 
current region being scanned was already there in `ClientScanner`.
    - From HBase 3 onwards, scan metrics for NSRE count and milliseconds spent 
b/w two consecutive next() calls, are not used. But in HBase 2 both above 
metrics are used so, they also had to be initialized in 
`ScanMetrics.moveToNextRegion()` and ITs had to be adjusted accordingly.


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