synchronized void processMetrics() {
                for (KafkaMetric metric : activeMetrics.values()) {
                    assertNotNull(metric.metricValue(), "Invalid metric value");
                }
            }
Verifies that concurrent sensor add, remove, updates and read with a metrics reporter
that synchronizes on every reporter method doesn't result in errors or deadlock.
clients/src/test/java/org/apache/kafka/common/metrics/MetricsTest.java