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/middleware/request_tracker_spec.rb

@@ -661,6 +661,7 @@ RSpec.describe Middleware::RequestTracker do
       lambda do |env|
         sql_calls.times { User.where(id: -100).pluck(:id) }
         redis_calls.times { Discourse.redis.get("x") }
+        yield if block_given?
         result
       end
     end
@@ -749,6 +750,36 @@ RSpec.describe Middleware::RequestTracker do
       expect(headers["X-Runtime"].to_f).to be > 0
     end
 
+    it "correctly logs GC stats when `instrument_gc_stat_per_request` site setting has been enabled" do
+      tracker =
+        Middleware::RequestTracker.new(
+          app([200, {}, []]) do
+            GC.start(full_mark: true) # Major GC
+            GC.start(full_mark: false) # Minor GC
+          end,
+        )
+
+      tracker.call(env)
+
+      expect(@data[:timing][:gc]).to eq(nil)
+
+      SiteSetting.instrument_gc_stat_per_request = true
+
+      tracker =
+        Middleware::RequestTracker.new(
+          app([200, {}, []]) do
+            GC.start(full_mark: true) # Major GC
+            GC.start(full_mark: false) # Minor GC
+          end,
+        )
+
+      tracker.call(env)
+
+      expect(@data[:timing][:gc][:time]).to be > 0.0
+      expect(@data[:timing][:gc][:major_count]).to eq(1)
+      expect(@data[:timing][:gc][:minor_count]).to eq(1)
+    end
+
     it "can correctly log messagebus request types" do
       tracker = Middleware::RequestTracker.new(app([200, {}, []]))