mc update: init stages, better subsystem init tracking, etc (#6857)

modifies the hollow metrics API; that'll probably be implemented soon
adds init stages
changes SSatoms to not abuse its `initialized` variable

closes #6858

code/modules/metrics/api/string_set.dm

@@ -0,0 +1,17 @@
+//* This file is explicitly licensed under the MIT license. *//
+//* Copyright (c) 2024 Citadel Station Developers           *//
+
+/**
+ * Records an unique set of strings.
+ *
+ * This is what you can use for things like:
+ *
+ * * What subsystems failed init
+ */
+/datum/metric/string_set
+
+/**
+ * records a string into a string set
+ */
+/proc/metric_record_string_set(datum/metric/string_set/typepath, string)
+	return