Save artifacts sizes

What does this MR do?

Introduce ci_builds.artifacts_size as an integer, so that it's easier to access than reading from the file again.

What are the relevant issue numbers?

Closes #18869 (closed)

app/models/ci/build.rb

@@ -19,6 +19,7 @@ module Ci
     acts_as_taggable
+    before_save :update_artifacts_size, if: :artifacts_file_changed?
     before_destroy { project }
     after_create :execute_hooks
@@ -329,7 +330,12 @@ module Ci
     end
     def artifacts_metadata_entry(path, **options)
-      Gitlab::Ci::Build::Artifacts::Metadata.new(artifacts_metadata.path, path, **options).to_entry
+      metadata = Gitlab::Ci::Build::Artifacts::Metadata.new(
+        artifacts_metadata.path,
+        path,
+        **options)
+
+      metadata.to_entry
     end
     def erase_artifacts!
@@ -375,6 +381,14 @@ module Ci
     private
+    def update_artifacts_size
+      self.artifacts_size = if artifacts_file.exists?
+                              artifacts_file.size
+                            else
+                              nil
+                            end
+    end
+
     def erase_trace!
       self.trace = nil
     end