Add Benchmark.ms method and enhance realtime with unit parameter

Rails previously included a monkeypatch that added a `ms` method to the
Benchmark module to return timing results in milliseconds instead of
seconds. This monkeypatch has been deprecated in Rails and will be
removed in future versions. ref: rails/rails@4cdf757

This commit adds native support for millisecond timing measurements by:

1. **Enhanced Benchmark.realtime method**: Now accepts an optional `unit`
   parameter (defaulting to `:float_second` for backward compatibility)
   that leverages `Process.clock_gettime`'s built-in unit support.

2. **New Benchmark.ms method**: A convenience method that returns elapsed
   time in milliseconds, equivalent to `realtime(:float_millisecond)`.

For web applications, measuring performance in milliseconds is much more helpful since the majority of operations happen in less than a second.
While it's acceptable to display times >1s as "1.2s", showing "0.125s" instead of "125ms" is significantly less readable and intuitive for
developers analyzing performance metrics.

```ruby
Benchmark.realtime { sleep 0.1 }  #=> 0.10023

Benchmark.realtime(:float_millisecond) { sleep 0.1 }  #=> 100.23

Benchmark.ms { sleep 0.1 }  #=> 100.23
```

This change provides a clean migration path for Rails applications
currently relying on the deprecated monkeypatch while offering
enhanced functionality for all benchmark users.

As an alternative approach, the .realtime method now accepts an optional
unit argument that gets passed directly to Process.clock_gettime, allowing
developers to specify their preferred time unit (:float_second, :float_millisecond,
:float_microsecond, :nanosecond) while maintaining backward compatibility.

Author: nvasilevski

lib/benchmark.rb

@@ -314,18 +314,33 @@ def measure(label = "") # :yield:
 
   #
   # Returns the elapsed real time used to execute the given block.
-  # The unit of time is seconds.
+  # The unit of time is seconds by default, or can be specified with the unit parameter.
+  # See documentation for `Process.clock_gettime`` for supported units.
   #
   #       Benchmark.realtime { "a" * 1_000_000_000 }
   #       #=> 0.5098029999935534
   #
-  def realtime # :yield:
-    r0 = Process.clock_gettime(Process::CLOCK_MONOTONIC)
+  #       Benchmark.realtime(:float_millisecond) { "a" * 1_000_000_000 }
+  #       #=> 509.8029999935534
+  #
+  def realtime(unit = :float_second) # :yield:
+    r0 = Process.clock_gettime(Process::CLOCK_MONOTONIC, unit)
     yield
-    Process.clock_gettime(Process::CLOCK_MONOTONIC) - r0
+    Process.clock_gettime(Process::CLOCK_MONOTONIC, unit) - r0
+  end
+
+  #
+  # Returns the elapsed real time used to execute the given block.
+  # The unit of time is milliseconds.
+  #
+  #       Benchmark.ms { "a" * 1_000_000_000 }
+  #       #=> 509.8029999935534
+  #
+  def ms # :yield:
+    realtime(:float_millisecond) { yield }
   end
 
-  module_function :benchmark, :measure, :realtime, :bm, :bmbm
+  module_function :benchmark, :measure, :realtime, :ms, :bm, :bmbm
 
   #
   # A Job is a sequence of labelled blocks to be processed by the