pubsub: Pubsub offers no Close method for closing BulkIndexer

[axw]

The tail-based sampling pubsub implementation internally uses a go-elasticsearch BulkIndexer for indexing sampling decisions. This is never closed, as there is no method for closing the pubsub:

apm-server/x-pack/apm-server/sampling/pubsub/pubsub.go

Lines 54 to 60 in 7b86cb7

	indexer, err := config.Client.NewBulkIndexer(elasticsearch.BulkIndexerConfig{
	Index: config.DataStream.String(),
	FlushInterval: config.FlushInterval,
	OnError: func(ctx context.Context, err error) {
	config.Logger.With(logp.Error(err)).Debug("publishing sampled trace IDs failed")
	},
	})