Use producer counter for confirm rate

Fixes #102

Author: acogoluegnes

src/main/java/com/rabbitmq/stream/perf/DefaultPerformanceMetrics.java

@@ -116,7 +116,7 @@ public void start(String description) throws Exception {
     long startTime = System.nanoTime();
 
     String metricPublished = metricsName("published");
-    String metricProducerConfirmed = metricsName("confirmed");
+    String metricProducerConfirmed = metricsName("producer_confirmed");
     String metricConsumed = metricsName("consumed");
     String metricChunkSize = metricsName("chunk_size");
     String metricLatency = metricsName("latency");