Array type coercion (arrays are covariant)

prometheus_patch.txt

diff --git a/core/src/main/java/com/linecorp/armeria/internal/metric/PrometheusMetricRequestDecorator.java b/core/src/main/java/com/linecorp/armeria/internal/metric/PrometheusMetricRequestDecorator.java
index 29a4523d..ce71ae60 100644
--- a/core/src/main/java/com/linecorp/armeria/internal/metric/PrometheusMetricRequestDecorator.java
+++ b/core/src/main/java/com/linecorp/armeria/internal/metric/PrometheusMetricRequestDecorator.java
@@ -24,8 +24,8 @@ import java.util.concurrent.TimeUnit;
 import java.util.function.Function;
 import java.util.stream.Stream;
 
-import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableSortedMap;
+import com.google.common.collect.Iterables;
 
 import com.linecorp.armeria.client.Client;
 import com.linecorp.armeria.client.ClientRequestContext;
@@ -144,24 +144,24 @@ public final class PrometheusMetricRequestDecorator<T extends MetricLabel<T>,
     }
 
     private final Function<RequestLog, Map<T, String>> labelingFunction;
-    private final MetricAdapter metricAdapter;
+    private final MetricAdapter<T> metricAdapter;
 
     private PrometheusMetricRequestDecorator(
             CollectorRegistry collectorRegistry,
-            Object[] metricLabels,
+            T[] metricLabels,
             Function<RequestLog, Map<T, String>> labelingFunction,
             String prefix) {
         requireNonNull(collectorRegistry, "collectorRegistry");
         requireNonNull(metricLabels, "metricLabels");
-        Object[] metricLabelsCopy = Arrays.copyOf(metricLabels, metricLabels.length);
-        for (Object metricLabel : metricLabelsCopy) {
+        T[] metricLabelsCopy = Arrays.copyOf(metricLabels, metricLabels.length);
+        for (T metricLabel : metricLabelsCopy) {
             requireNonNull(metricLabel, "metricLabels contains null");
         }
         requireNonNull(labelingFunction, "labelingFunction");
         requireNonNull(prefix, "prefix");
 
         this.labelingFunction = labelingFunction;
-        this.metricAdapter = new MetricAdapter(collectorRegistry, metricLabelsCopy, prefix, HELP_STRING);
+        this.metricAdapter = new MetricAdapter<>(collectorRegistry, metricLabelsCopy, prefix, HELP_STRING);
     }
 
     private PrometheusMetricRequestDecorator(
@@ -170,7 +170,7 @@ public final class PrometheusMetricRequestDecorator<T extends MetricLabel<T>,
             Function<RequestLog, Map<T, String>> labelingFunction,
             String prefix) {
         this(collectorRegistry,
-             ImmutableList.copyOf(requireNonNull(metricLabels, "metricLabels")).toArray(),
+             (T[]) Iterables.toArray(requireNonNull(metricLabels, "metricLabels"), MetricLabel.class),
              labelingFunction,
              prefix);
     }
@@ -255,7 +255,7 @@ public final class PrometheusMetricRequestDecorator<T extends MetricLabel<T>,
         O apply(C c, I i) throws Exception;
     }
 
-    private static final class MetricAdapter {
+    private static final class MetricAdapter<T extends MetricLabel<T>> {
 
         private final Summary timer;
         private final Counter success;
@@ -265,7 +265,7 @@ public final class PrometheusMetricRequestDecorator<T extends MetricLabel<T>,
         private final Summary responseBytes;
 
         private MetricAdapter(CollectorRegistry collectorRegistry,
-                              Object[] metricLabels,
+                              T[] metricLabels,
                               String namePrefix,
                               String help) {
             requireNonNull(collectorRegistry, "collectorRegistry");