Lower multipart log threshold from 500 MB to 100 MB

This also makes the threshold configurable via an environment variable
`RACK_MULTIPART_LOGGING_BYTES`.

This will help identify uploads in
https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/12802.

config/initializers/rack_multipart_patch.rb

@@ -13,7 +13,7 @@ module Rack
         def log_multipart_warning(req)
           content_length = req.content_length.to_i
 
-          return unless content_length > 500_000_000
+          return unless content_length > log_threshold
 
           message = {
             message: "Large multipart body detected",
@@ -32,6 +32,10 @@ module Rack
         def log_large_multipart?
           Gitlab::Utils.to_boolean(ENV['ENABLE_RACK_MULTIPART_LOGGING'], default: true) && Gitlab.com?
         end
+
+        def log_threshold
+          ENV.fetch('RACK_MULTIPART_LOGGING_BYTES', 100_000_000).to_i
+        end
       end
 
       prepend MultipartPatch

spec/initializers/rack_multipart_patch_spec.rb

@@ -42,8 +42,9 @@ EOF
   end
 
   context 'with Content-Length over the limit' do
+    shared_examples 'logs multipart message' do
       it 'extracts multipart message' do
-      env = Rack::MockRequest.env_for("/", multipart_fixture(:text, 500_000_001))
+        env = Rack::MockRequest.env_for("/", multipart_fixture(:text, length))
 
         expect(described_class).to receive(:log_large_multipart?).and_return(true)
         expect(described_class).to receive(:log_multipart_warning).and_call_original
@@ -58,4 +59,21 @@ EOF
         expect(params.keys).to include(*%w(reply fileupload))
       end
     end
+
+    context 'from environment' do
+      let(:length) { 1001 }
+
+      before do
+        stub_env('RACK_MULTIPART_LOGGING_BYTES', 1000)
+      end
+
+      it_behaves_like 'logs multipart message'
+    end
+
+    context 'default limit' do
+      let(:length) { 100_000_001 }
+
+      it_behaves_like 'logs multipart message'
+    end
+  end
 end