hrpc: enable ScanMetrics

[ciacono]

Enable tracking scan metrics in the ScanResponse. Clients can access the metrics via calls to scanner.Next()
Support for HBase versions < 2.6.0 where ScanMetrics provides ROWS_SCANNED and ROWS_FILTERED metrics.

[dethi]

Left a few comments but I also wonder if adding the scan metrics to the hrpc.Results is the right choice here. It seems that we will be allocating a lot of small struct on the heap to hold these result and most likely the user of this feature wants to know the overall scan metrics at the end so they will have to sum the various result in their own code.

Would it make more sense to hold the scan metrics in the scanner struct itself, accumulating the result as we fetch and add a method to the scanner to returns the end metrics. And also doing that while keepting the scan metrics as map[string]int64 such that there is no dependencies with the HBase version at all (if a new version of HBase introduce a new scan metrics name, or change one, we don't need to update GoHBase accordintly).

What are your thoughts on this?

[tsuna]

I echo Thibault's feedback here that always passing around the scan metrics along with the batch of results makes the code more complex while also making the scan metrics harder to use from the caller's point of view. Not sure we should keep them in a generic map[string]int64 as this would require assuming we can always accumulate all the metrics by summing them up, but either way I do agree we should track the metrics directly in the scan object instead of passing them around all the time separately, so that the caller can consult the cumulative metrics at the end of the scan, which will most likely be the most common use case.

[dethi]

LGTM 🚢