BulkIndexer WaitGroup delta is added after workers are launched

[Jakub-Kapusta]

go-elasticsearch/esutil/bulk_indexer.go

Lines 380 to 383 in a58e89f

	w.run()
	bi.workers = append(bi.workers, &w)
	}
	bi.wg.Add(bi.config.NumWorkers)

The wg Delta should generally be added before relevant go-routines are launched.
Reference: https://pkg.go.dev/sync#WaitGroup.Add

This might not be a big issue in this case, but it strays from best practices.
Also, it creates a small window where a worker could call Done() before the call to Add(), which is not ideal.