fix: Pass metric_prefix to Aggregator finalizer

CHANGELOG.md

@@ -6,6 +6,7 @@ section below.
 
 ## Unreleased changes
 
+- [#416](https://github.com/Shopify/statsd-instrument/pull/416) - Fix missing `metric_prefix` in Aggregator finalizer, causing metrics to lose their prefix when flushed during GC.
 - [#415](https://github.com/Shopify/statsd-instrument/pull/415) - Fix `sample_rate` being ignored when aggregation is enabled. Previously, `increment`, `measure`, and `histogram` calls would bypass sampling entirely when aggregation was enabled, causing metrics to be emitted at 100% rate regardless of the configured sample rate.
 - [#411](https://github.com/Shopify/statsd-instrument/pull/403) - Add `CompiledMetric::Gauge` as the second metric type to support pre-compiled metric datagrams. It can be used as a replacement over standard `StatsD.gauge`.
 - Add support for `nil` tag values in `CompiledMetric` dynamic tags (converted to empty string).

lib/statsd/instrument/aggregator.rb

@@ -133,6 +133,7 @@ def initialize(
           @datagram_builders,
           @datagram_builder_class,
           @default_tags,
+          @metric_prefix,
         )
 
         ObjectSpace.define_finalizer(

test/aggregator_test.rb

@@ -216,6 +216,31 @@ def test_with_prefix
     assert_equal(1, @sink.datagrams.last.value)
   end
 
+  def test_finalizer_with_prefix
+    # Test that the finalizer correctly uses the prefix when flushing metrics
+    # IMPORTANT: Use empty tags to ensure datagram_builder is not pre-created by tags_sorted
+    aggregator = StatsD::Instrument::Aggregator.new(@sink, StatsD::Instrument::DatagramBuilder, "MyApp", [])
+
+    aggregator.increment("foo", 1, tags: [])
+    aggregator.increment("bar", 1, tags: [], no_prefix: true)
+
+    # Manually trigger the finalizer (simulates GC cleanup)
+    finalizer = StatsD::Instrument::Aggregator.finalize(
+      aggregator.instance_variable_get(:@finalizer),
+    )
+    finalizer.call
+
+    assert_equal(2, @sink.datagrams.size)
+
+    prefixed_datagram = @sink.datagrams.find { |d| d.name == "MyApp.foo" }
+    refute_nil(prefixed_datagram, "Expected to find datagram with prefix 'MyApp.foo'")
+    assert_equal(1, prefixed_datagram.value)
+
+    unprefixed_datagram = @sink.datagrams.find { |d| d.name == "bar" }
+    refute_nil(unprefixed_datagram, "Expected to find datagram without prefix 'bar'")
+    assert_equal(1, unprefixed_datagram.value)
+  end
+
   def test_synchronous_operation_on_thread_failure
     # Force thread_healthcheck to return false
     @subject.stubs(:thread_healthcheck).returns(false)