DEV: Seperate concerns of tracking GC stat from MethodProfiler (#22921)

Why this change?

This is a follow up to e8f7b62752a65e33aa1cee748fc966c4ad14b1ee.
Tracking of GC stats didn't really belong in the `MethodProfiler` class
so we want to extract that concern into its own class.

As part of this PR, the `track_gc_stat_per_request` site setting has
also been renamed to `instrument_gc_stat_per_request`.

spec/lib/gc_stat_instrumenter_spec.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+RSpec.describe GCStatInstrumenter do
+  describe ".instrument" do
+    it "returns a hash with the GC time, major and minor GC count for executing the given block" do
+      result =
+        described_class.instrument do
+          GC.start(full_mark: true) # Major GC
+          GC.start(full_mark: false) # Minor GC
+        end
+
+      expect(result[:gc]).to be_present
+      expect(result[:gc][:time]).to be >= 0.0
+      expect(result[:gc][:major_count]).to eq(1)
+      expect(result[:gc][:minor_count]).to eq(1)
+    end
+  end
+end