Allow GC to collect unneeded slice elements (#5804)

```go
type interInst struct {
	x int
}

type inter interface {
}

var sink []inter

func BenchmarkX(b *testing.B) {
	sink = make([]inter, b.N)
	for i := 0; i < b.N; i++ {
		sink[i] = &interInst{}
	}
	clear(sink)
	sink = sink[:0]
	runtime.GC()
	var ms runtime.MemStats
	runtime.ReadMemStats(&ms)
	b.Log(b.N, ms.Frees) // Frees is the cumulative count of heap objects freed.
}
```

```
clear:
    ioz_test.go:35: 1 589
    ioz_test.go:35: 100 711
    ioz_test.go:35: 10000 10729
    ioz_test.go:35: 1000000 1010750  <-- 1m+ freed
    ioz_test.go:35: 16076874 17087643
    ioz_test.go:35: 19514749 36602412
```
```
no clear:
    ioz_test.go:35: 1 585
    ioz_test.go:35: 100 606
    ioz_test.go:35: 10000 725
    ioz_test.go:35: 1000000 10745  <-- some "overhead" objects freed, not the slice.
    ioz_test.go:35: 16391445 1010765
    ioz_test.go:35: 21765238 17402230
```

This is documented at https://go.dev/wiki/SliceTricks:

> NOTE If the type of the element is a pointer or a struct with pointer
fields, which need to be garbage collected, the above implementations of
Cut and Delete have a potential memory leak problem: some elements with
values are still referenced by slice a’s underlying array, just not
“visible” in the slice. Because the “deleted” value is referenced in the
underlying array, the deleted value is still “reachable” during GC, even
though the value cannot be referenced by your code. If the underlying
array is long-lived, this represents a leak.

Followed by examples of how zeroing out the slice elements solves
the problem. This PR does the same.

sdk/trace/batch_span_processor.go

@@ -280,6 +280,7 @@ func (bsp *batchSpanProcessor) exportSpans(ctx context.Context) error {
 		//
 		// It is up to the exporter to implement any type of retry logic if a batch is failing
 		// to be exported, since it is specific to the protocol and backend being sent to.
+		clear(bsp.batch) // Erase elements to let GC collect objects
 		bsp.batch = bsp.batch[:0]
 
 		if err != nil {