Improved Meter.Id#getTags() performance (#6182)

Avoids allocating an ArrayList for empty Tags, and sizes the ArrayList with new package-private API on Tags to get the size. This avoids the cost of growing the ArrayList or allocating a bigger array than needed.

Signed-off-by: etki <[email protected]>
Co-authored-by: Tommy Ludwig <[email protected]>

Author: etki

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

@@ -281,9 +281,15 @@ public String getName() {
          * @return A set of dimensions that allows you to break down the name.
          */
         public List<Tag> getTags() {
-            List<Tag> tags = new ArrayList<>();
-            this.tags.forEach(tags::add);
-            return Collections.unmodifiableList(tags);
+            if (this.tags == Tags.empty()) {
+                return Collections.emptyList();
+            }
+
+            List<Tag> list = new ArrayList<>(this.tags.size());
+            for (Tag tag : this.tags) {
+                list.add(tag);
+            }
+            return Collections.unmodifiableList(list);
         }
 
         public Iterable<Tag> getTagsAsIterable() {

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

@@ -225,6 +225,15 @@ public Tags and(@Nullable Iterable<? extends Tag> tags) {
         return merge(Tags.of(tags));
     }
 
+    /**
+     * Non-public (for now) method to get the size of this, which can be useful in sizing
+     * a collection where these elements will be copied.
+     * @return number of unique {@link Tag} instances in this
+     */
+    int size() {
+        return length;
+    }
+
     @Override
     public Iterator<Tag> iterator() {
         return new ArrayIterator();