[chore] Nit-fix exporter.request.MergeSplit() (#12016)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description

This PR fixes how `capacityLimit` is updated in
`exporter.request.MergeSplit()`.

Before
```
		if srcReq.ld.LogRecordCount() <= capacityLeft {
			if destReq == nil {
				destReq = srcReq
			} else {
				srcReq.ld.ResourceLogs().MoveAndAppendTo(destReq.ld.ResourceLogs())
			}
			capacityLeft -= destReq.ld.LogRecordCount()
			continue
		}
```

After

```
		srcCount := srcReq.ld.LogRecordCount()
		if srcCount <= capacityLeft {
			if destReq == nil {
				destReq = srcReq
			} else {
				srcReq.ld.ResourceLogs().MoveAndAppendTo(destReq.ld.ResourceLogs())
			}
			capacityLeft -= srcCount
			continue
		}
```

With that said, the original implementation does not cause any bug
because "the larger for loop" loops through only two items. In the first
loop, `destReq` is guaranteed to be `nil`, so `capacityLeft` is updated
properly. In the second loop, we jump out of the loop immediately so the
accuracy of `capacityLeft` does not matter any more.

<!-- Issue number if applicable -->
#### Link to tracking issue
NA

<!--Describe what testing was performed and which tests were added.-->
#### Testing
NA

<!--Describe the documentation added.-->
#### Documentation
NA
<!--Please delete paragraphs that you did not use before submitting.-->

exporter/exporterhelper/logs_batch.go

@@ -37,13 +37,15 @@ func (req *logsRequest) MergeSplit(_ context.Context, cfg exporterbatcher.MaxSiz
 		if srcReq == nil {
 			continue
 		}
-		if srcReq.ld.LogRecordCount() <= capacityLeft {
+
+		srcCount := srcReq.ld.LogRecordCount()
+		if srcCount <= capacityLeft {
 			if destReq == nil {
 				destReq = srcReq
 			} else {
 				srcReq.ld.ResourceLogs().MoveAndAppendTo(destReq.ld.ResourceLogs())
 			}
-			capacityLeft -= destReq.ld.LogRecordCount()
+			capacityLeft -= srcCount
 			continue
 		}
 

exporter/exporterhelper/metrics_batch.go

@@ -37,13 +37,15 @@ func (req *metricsRequest) MergeSplit(_ context.Context, cfg exporterbatcher.Max
 		if srcReq == nil {
 			continue
 		}
-		if srcReq.md.DataPointCount() <= capacityLeft {
+
+		srcCount := srcReq.md.DataPointCount()
+		if srcCount <= capacityLeft {
 			if destReq == nil {
 				destReq = srcReq
 			} else {
 				srcReq.md.ResourceMetrics().MoveAndAppendTo(destReq.md.ResourceMetrics())
 			}
-			capacityLeft -= destReq.md.DataPointCount()
+			capacityLeft -= srcCount
 			continue
 		}
 

exporter/exporterhelper/traces_batch.go

@@ -37,13 +37,15 @@ func (req *tracesRequest) MergeSplit(_ context.Context, cfg exporterbatcher.MaxS
 		if srcReq == nil {
 			continue
 		}
-		if srcReq.td.SpanCount() <= capacityLeft {
+
+		srcCount := srcReq.td.SpanCount()
+		if srcCount <= capacityLeft {
 			if destReq == nil {
 				destReq = srcReq
 			} else {
 				srcReq.td.ResourceSpans().MoveAndAppendTo(destReq.td.ResourceSpans())
 			}
-			capacityLeft -= destReq.td.SpanCount()
+			capacityLeft -= srcCount
 			continue
 		}
 

exporter/exporterhelper/xexporterhelper/profiles_batch.go

@@ -37,13 +37,15 @@ func (req *profilesRequest) MergeSplit(_ context.Context, cfg exporterbatcher.Ma
 		if srcReq == nil {
 			continue
 		}
-		if srcReq.pd.SampleCount() <= capacityLeft {
+
+		srcCount := srcReq.pd.SampleCount()
+		if srcCount <= capacityLeft {
 			if destReq == nil {
 				destReq = srcReq
 			} else {
 				srcReq.pd.ResourceProfiles().MoveAndAppendTo(destReq.pd.ResourceProfiles())
 			}
-			capacityLeft -= destReq.pd.SampleCount()
+			capacityLeft -= srcCount
 			continue
 		}