Pull batcher does not correctly count the done/index to signal on terminate

[bogdandrutu]

The bug was before the conversion to done callback, but here I will show the links after done callback change.

• If an existing batch is current with data, and a new request comes that makes the batch to exceed the minimum limit or getting split the previous done callback is not called.

See

opentelemetry-collector/exporter/internal/queue/default_batcher.go

Line 68 in eb4f640

qb.flush(ctx, reqList[i], []exporterqueue.DoneCallback{done})

even if the reqList is created

opentelemetry-collector/exporter/internal/queue/default_batcher.go

Line 54 in eb4f640

reqList, mergeSplitErr = qb.currentBatch.req.MergeSplit(ctx, qb.batchCfg.MaxSizeConfig, req)

we don't signal existing "dones".