Efficient copy of meters to list

Gets rid of an extra copy, which generates garbage for no benefit.
The new implementation is at least as performant as the previous one while generating approximately half the garbage.

Resolves gh-7035
Supersedes gh-7040

Author: shakuzen

micrometer-core/src/main/java/io/micrometer/core/instrument/MeterRegistry.java

@@ -408,10 +408,18 @@ Meter register(Meter.Id id, Meter.Type type, Iterable<Measurement> measurements)
     }
 
     /**
-     * @return The set of registered meters.
+     * Get currently registered meters.
+     * @return An unmodifiable copy of registered meters.
+     * @implNote Avoids the ArrayList constructor because it copies the array
+     * twice.
      */
+    @SuppressWarnings("unchecked")
     public List<Meter> getMeters() {
-        return Collections.unmodifiableList(new ArrayList<>(meterMap.values()));
+        return Collections.unmodifiableList(
+                // Arrays.asList wraps the array without copying it (unlike new ArrayList)
+                // The cast is safe because we know the map only contains Meters and the
+                // returned list is unmodifiable
+                (List<Meter>) (List<?>) Arrays.asList(meterMap.values().toArray()));
     }
 
     /**