fix: Pass metric_prefix to Aggregator finalizer

[kelhusseiny]

Summary

Fixes a bug where metric_prefix was not passed to the Finalizer struct, causing metrics to lose their prefix when flushed during garbage collection.

Root Cause

The Finalizer struct expects 6 arguments:

Finalizer = Struct.new(:aggregation_state, :sink, :datagram_builders, :datagram_builder_class, :default_tags, :metric_prefix)

But only 5 were passed:

@finalizer = Finalizer.new(
  @aggregation_state,
  @sink,
  @datagram_builders,
  @datagram_builder_class,
  @default_tags,
  # metric_prefix was missing!
)

This left metric_prefix as nil, so when the finalizer flushed metrics during GC, they would be emitted without the configured prefix.

When the bug manifests

The bug only occurs when all these conditions are met:

1. Aggregator is created with a prefix (e.g., "MyApp")
2. Metrics are added without tags (tags trigger datagram_builder creation via tags_sorted)
3. GC runs before normal flush, triggering the finalizer

When tags are present, tags_sorted creates the datagram_builder with the correct prefix, masking the bug.

Test plan

• Added test_finalizer_with_prefix that verifies the finalizer preserves metric prefix
• Test uses empty tags to ensure datagram_builder is not pre-created
• Test fails without fix, passes with fix
• All existing tests pass (370 runs, 932 assertions)