Make KafkaMetrics class public

`KafkaMetrics` class is currently _package private_.
Making this public will enable using it in cases where constructing `KafkaClientMetrics` nor `KafkaConsumerMetrics` is a viable option.

Signed-off-by: Ondra Pelech <[email protected]>

Author: sideeffffect

micrometer-core/src/main/java/io/micrometer/core/instrument/binder/kafka/KafkaMetrics.java

@@ -56,7 +56,7 @@
 @Incubating(since = "1.4.0")
 @NonNullApi
 @NonNullFields
-class KafkaMetrics implements MeterBinder, AutoCloseable {
+public class KafkaMetrics implements MeterBinder, AutoCloseable {
 
     private static final InternalLogger log = InternalLoggerFactory.getInstance(KafkaMetrics.class);