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/ewma.rb

@@ -0,0 +1,66 @@
+# Vendored from metriks 0.9.9.8 — https://github.com/eric/metriks
+# Copyright (c) 2012 Eric Lindvall, MIT licensed.
+# Trimmed to the subset dyndnsd uses (meters + registry + reporters).
+
+
+module Metriks
+  class EWMA
+    INTERVAL = 5.0
+    SECONDS_PER_MINUTE = 60.0
+
+    ONE_MINUTE      = 1
+    FIVE_MINUTES    = 5
+    FIFTEEN_MINUTES = 15
+
+    M1_ALPHA  = 1 - Math.exp(-INTERVAL / SECONDS_PER_MINUTE / ONE_MINUTE)
+    M5_ALPHA  = 1 - Math.exp(-INTERVAL / SECONDS_PER_MINUTE / FIVE_MINUTES)
+    M15_ALPHA = 1 - Math.exp(-INTERVAL / SECONDS_PER_MINUTE / FIFTEEN_MINUTES)
+
+    def self.new_m1
+      new(M1_ALPHA, INTERVAL)
+    end
+
+    def self.new_m5
+      new(M5_ALPHA, INTERVAL)
+    end
+
+    def self.new_m15
+      new(M15_ALPHA, INTERVAL)
+    end
+
+    def initialize(alpha, interval)
+      @alpha    = alpha
+      @interval = interval
+
+      @initialized = false
+      @rate        = Atomic.new(0.0)
+      @uncounted   = Atomic.new(0)
+    end
+
+    def clear
+      @initialized = false
+      @rate.value = 0.0
+      @uncounted.value = 0
+    end
+
+    def update(value)
+      @uncounted.update { |v| v + value }
+    end
+
+    def tick
+      count = @uncounted.swap(0)
+      instant_rate = count / @interval.to_f
+
+      if @initialized
+        @rate.update { |v| v + @alpha * (instant_rate - v) }
+      else
+        @rate.value = instant_rate
+        @initialized = true
+      end
+    end
+
+    def rate
+      @rate.value
+    end
+  end
+end