feat: add readBatch span (#492)

* feat: add readBatch span

we've observed large 'Stream' traces with a small semaphore or processor
usage.
To improve clarity also trace readBatch since it was observed causing slow
requests in some instances

* Update processor.go

Author: kruskall

input/elasticapm/processor.go

@@ -179,6 +179,8 @@ func (p *Processor) readBatch(
 	reader *streamReader,
 	result *Result,
 ) (int, error) {
+	_, sp := p.tracer.Start(ctx, "readBatch")
+	defer sp.End()
 
 	// input events are decoded and appended to the batch
 	origLen := len(*batch)