gems: include vendored metrisk without hitime dependency

hitime is only needed for Metriks::Timer and requires native extensions.
Those do not compile with default settings under Ubuntu 26.04 or Alpine 3.24.

lib/metriks/atomic.rb

@@ -0,0 +1,34 @@
+# Minimal pure-Ruby replacement for the 'atomic' gem, covering the exact API
+# the vendored metriks code uses (Atomic.new, #value, #value=, #update,
+# #compare_and_swap). Mutex-guarded instead of relying on a native extension.
+module Metriks
+  class Atomic
+    def initialize(value = nil)
+      @value = value
+      @mutex = Mutex.new
+    end
+
+    def value
+      @mutex.synchronize { @value }
+    end
+
+    def value=(new_value)
+      @mutex.synchronize { @value = new_value }
+    end
+
+    def update
+      @mutex.synchronize { @value = yield(@value) }
+    end
+
+    def compare_and_swap(expected, new_value)
+      @mutex.synchronize do
+        if @value == expected
+          @value = new_value
+          true
+        else
+          false
+        end
+      end
+    end
+  end
+end