Expose the global direct memory counter. (#8945)

Motivation:
This counter is very useful in order to monitor Netty without having every ByteBufAllocator in the JVM

Modification:
Expose the value of DIRECT_MEMORY_COUNTER as we are already doing for DIRECT_MEMORY_LIMIT.
We are returning -1 in case that DIRECT_MEMORY_COUNTER is not available.

Result:

Be able to get the amount of direct memory used.

common/src/main/java/io/netty/util/internal/PlatformDependent.java

@@ -263,6 +263,16 @@ public final class PlatformDependent {
         return DIRECT_MEMORY_LIMIT;
     }
 
+    /**
+     * Returns the current memory reserved for direct buffer allocation.
+     * This method returns -1 in case that a value is not available.
+     *
+     * @see #maxDirectMemory()
+     */
+    public static long usedDirectMemory() {
+        return DIRECT_MEMORY_COUNTER != null ? DIRECT_MEMORY_COUNTER.get() : -1;
+    }
+
     /**
      * Returns the temporary directory.
      */