add benchmark rake task

tasks/benchmark.rake

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+require 'benchmark'
+require 'prometheus/client'
+
+task :increment do
+  n = 1_000_000
+  counter = Prometheus::Client::Counter.new(:counter, 'Benchmark counter')
+
+  Benchmark.bm do |x|
+    x.report { n.times { counter.increment; } }
+    x.report { n.times { counter.increment({}, 1, '', ''); } }
+  end
+end