Concurrent.monotonic_time accept an optional unit parameter

This is a direct transposition of MRI's `clock_gettime` unit option.

lib/concurrent-ruby/concurrent/utility/monotonic_time.rb

@@ -10,25 +10,54 @@ def initialize
 
     if defined?(Process::CLOCK_MONOTONIC)
       # @!visibility private
-      def get_time
-        Process.clock_gettime(Process::CLOCK_MONOTONIC)
+      def get_time(unit)
+        Process.clock_gettime(Process::CLOCK_MONOTONIC, unit)
       end
     elsif Concurrent.on_jruby?
       # @!visibility private
-      def get_time
-        java.lang.System.nanoTime() / 1_000_000_000.0
+      TIME_UNITS = Hash.new { |_hash, key| raise ArgumentError, "unexpected unit: #{key}" }.compare_by_identity
+      private_constant :TIME_UNITS
+      TIME_UNITS.merge!(
+        second: 1_000_000_000,
+        millisecond: 1_000_000,
+        microsecond: 1_000,
+        nanosecond: 1,
+        float_second: 1_000_000_000.0,
+        float_millisecond: 1_000_000.0,
+        float_microsecond: 1_000.0,
+      )
+
+      # @!visibility private
+      def get_time(unit)
+        java.lang.System.nanoTime() / TIME_UNITS[unit]
       end
     else
+      # @!visibility private
+      TIME_UNITS = Hash.new { |_hash, key| raise ArgumentError, "unexpected unit: #{key}" }.compare_by_identity
+      private_constant :TIME_UNITS
+      TIME_UNITS.merge!(
+        second: [nil, true],
+        millisecond: [1_000, true],
+        microsecond: [1_000_000, true],
+        nanosecond: [1_000_000_000, true],
+        float_second: [nil, false],
+        float_millisecond: [1_000.0, false],
+        float_microsecond: [1_000_000.0, false],
+      )
 
       # @!visibility private
-      def get_time
+      def get_time(unit)
         synchronize do
           now = Time.now.to_f
           if @last_time < now
             @last_time = now
           else # clock has moved back in time
             @last_time += 0.000_001
           end
+          scale, to_int = TIME_UNITS[unit]
+          now *= scale if scale
+          now = now.to_i if to_int
+          now
         end
       end
 
@@ -46,12 +75,16 @@ def get_time
   #
   #   Returns the current time a tracked by the application monotonic clock.
   #
+  #   @param [Symbol] unit the time unit to be returned, can be either
+  #     :float_second, :float_millisecond, :float_microsecond, :second,
+  #     :millisecond, :microsecond, or :nanosecond default to :float_second.
+  #
   #   @return [Float] The current monotonic time since some unspecified
   #     starting point
   #
   #   @!macro monotonic_clock_warning
-  def monotonic_time
-    GLOBAL_MONOTONIC_CLOCK.get_time
+  def monotonic_time(unit = :float_second)
+    GLOBAL_MONOTONIC_CLOCK.get_time(unit)
   end
 
   module_function :monotonic_time

spec/concurrent/monotonic_time_spec.rb

@@ -0,0 +1,30 @@
+module Concurrent
+
+  RSpec.describe :monotonic_time do
+    context 'behavior' do
+
+      it 'returns seconds as float' do
+        expect(Concurrent.monotonic_time).to be_a(Float)
+      end
+
+      %i(float_second float_millisecond float_microsecond).each do |unit|
+        it "returns a Float when unit = #{unit.inspect}" do
+          expect(Concurrent.monotonic_time(unit)).to be_a(Float)
+        end
+      end
+
+      %i(second millisecond microsecond nanosecond).each do |unit|
+        it "returns an Integer when unit = #{unit.inspect}" do
+          expect(Concurrent.monotonic_time(unit)).to be_an(Integer)
+        end
+      end
+
+      it 'raises ArgumentError on unknown units' do
+        expect {
+          Concurrent.monotonic_time(:foo)
+        }.to raise_error(ArgumentError)
+      end
+
+    end
+  end
+end