MINOR: Add toString to various Kafka Metrics classes

clients/src/main/java/org/apache/kafka/common/metrics/Sensor.java

@@ -69,6 +69,11 @@ public Stat stat() {
         public MetricConfig config() {
             return configSupplier.get();
         }
+
+        @Override
+        public String toString() {
+            return "StatAndConfig(stat=" + stat + ')';
+        }
     }
 
     public enum RecordingLevel {

clients/src/main/java/org/apache/kafka/common/metrics/stats/CumulativeSum.java

@@ -47,4 +47,8 @@ public double measure(MetricConfig config, long now) {
         return total;
     }
 
+    @Override
+    public String toString() {
+        return "CumulativeSum(total=" + total + ")";
+    }
 }

clients/src/main/java/org/apache/kafka/common/metrics/stats/Frequency.java

@@ -56,4 +56,11 @@ public double centerValue() {
         return this.centerValue;
     }
 
+    @Override
+    public String toString() {
+        return "Frequency(" +
+            "name=" + name +
+            ", centerValue=" + centerValue +
+            ')';
+    }
 }

clients/src/main/java/org/apache/kafka/common/metrics/stats/Meter.java

@@ -82,4 +82,14 @@ public void record(MetricConfig config, double value, long timeMs) {
         double totalValue = (rate.stat instanceof WindowedCount) ? 1.0 : value;
         total.record(config, totalValue, timeMs);
     }
+
+    @Override
+    public String toString() {
+        return "Meter(" +
+            "rate=" + rate +
+            ", total=" + total +
+            ", rateMetricName=" + rateMetricName +
+            ", totalMetricName=" + totalMetricName +
+            ')';
+    }
 }

clients/src/main/java/org/apache/kafka/common/metrics/stats/Rate.java

@@ -94,6 +94,14 @@ public long windowSize(MetricConfig config, long now) {
         return totalElapsedTimeMs;
     }
 
+    @Override
+    public String toString() {
+        return "Rate(" +
+            "unit=" + unit +
+            ", stat=" + stat +
+            ')';
+    }
+
     /**
      * @deprecated since 2.4 Use {@link WindowedSum} instead.
      */

clients/src/main/java/org/apache/kafka/common/metrics/stats/SampledStat.java

@@ -93,6 +93,15 @@ public Sample oldest(long now) {
         return oldest;
     }
 
+    @Override
+    public String toString() {
+        return "SampledStat(" +
+            "initialValue=" + initialValue +
+            ", current=" + current +
+            ", samples=" + samples +
+            ')';
+    }
+
     protected abstract void update(Sample sample, MetricConfig config, double value, long timeMs);
 
     public abstract double combine(List<Sample> samples, MetricConfig config, long now);
@@ -128,6 +137,16 @@ public void reset(long now) {
         public boolean isComplete(long timeMs, MetricConfig config) {
             return timeMs - lastWindowMs >= config.timeWindowMs() || eventCount >= config.eventWindow();
         }
+
+        @Override
+        public String toString() {
+            return "Sample(" +
+                "value=" + value +
+                ", eventCount=" + eventCount +
+                ", lastWindowMs=" + lastWindowMs +
+                ", initialValue=" + initialValue +
+                ')';
+        }
     }
 
 }

clients/src/main/java/org/apache/kafka/common/metrics/stats/TokenBucket.java

@@ -98,4 +98,13 @@ private void refill(final double quota, final double burst, final long timeMs) {
     private double burst(final MetricConfig config) {
         return config.samples() * convert(config.timeWindowMs(), unit) * config.quota().bound();
     }
+
+    @Override
+    public String toString() {
+        return "TokenBucket(" +
+            "unit=" + unit +
+            ", tokens=" + tokens +
+            ", lastUpdateMs=" + lastUpdateMs +
+            ')';
+    }
 }