Automatically register the metics of PooledByteBufAllocator.DEFAULT (#5916)

Motivation:
Add metrics related to PooledByteBufAllocator that is already exposed by
the netty.

Modifications:
- `NettyAllocatorMetrics` are registered when MoreMeterBinders is
initialized.


Result:

- Fixes #2633
- Armeria now automatically registers the metics of
`PooledByteBufAllocator.DEFAULT`.

---------

Co-authored-by: Ikhun Um <ih.pert@gmail.com>

Author: Bue-von-hon

core/src/main/java/com/linecorp/armeria/common/metric/MoreMeterBinders.java

@@ -25,17 +25,26 @@
 
 import com.google.common.collect.ImmutableList;
 
+import com.linecorp.armeria.common.Flags;
 import com.linecorp.armeria.common.annotation.UnstableApi;
 import com.linecorp.armeria.internal.common.util.CertificateUtil;
 
 import io.micrometer.core.instrument.binder.MeterBinder;
+import io.micrometer.core.instrument.binder.netty4.NettyAllocatorMetrics;
+import io.netty.buffer.PooledByteBufAllocator;
 import io.netty.channel.EventLoopGroup;
 
 /**
  * Provides useful {@link MeterBinder}s to monitor various Armeria components.
  */
 public final class MoreMeterBinders {
 
+    static {
+        // Bind the default Netty allocator metrics to the default MeterRegistry.
+        new NettyAllocatorMetrics(PooledByteBufAllocator.DEFAULT)
+                .bindTo(Flags.meterRegistry());
+    }
+
     /**
      * Returns a new {@link MeterBinder} to observe Netty's {@link EventLoopGroup}s. The following stats are
      * currently exported per registered {@link MeterIdPrefix}.