[ART-380] Fix ArgumentError due to concurrent require and class use

If for some reason multiple tracers are initializing concurrently
(see customer issue in #1306 or the flaky test in
`tracer_integration_spec.rb` in the previous commit), multiple threads
can try to run `default_statsd_client` concurrently.

Because Ruby `require`s are not atomic (e.g. you can observe classes
that are half-loaded), our conditional

```ruby
require 'datadog/statsd' unless defined?(::Datadog::Statsd)
Datadog::Statsd.new(default_hostname, default_port)
```

could try to create a `Statsd` instance BEFORE the class was fully
defined, leading to
`ArgumentError: wrong number of arguments (given 2, expected 0)` when
calling `initialize`. This error stems from the custom `#initialize`
method not being available yet, so Ruby falls back to
`Object#initialize`, which does not take any argument.

See also:
* <https://slides.com/ianjo/spotting-unsafe-concurrent-ruby-patterns-fullstack-lx#/0/42/0>
* <https://gitlab.com/ivoanjo/unsafe-concurrent-ruby-patterns/-/blob/master/background_require.rb>

The fix in this case is to always call `require`. Calling `require`
repeatedly has little impact -- a given file is only loaded by
`require` the first time AND concurrent `require`s are protected --
only one thread will get to execute the require, will all others wait
until it finishes.

The follow example demonstrates this:

* `lazy-require.rb`:

```ruby
def try_require
  puts "#{Thread.current} Going to require"
  require_relative './lazy-require-sleep-1'
  puts "#{Thread.current} After require"
end

2.times.map { Thread.new { try_require } }.map(&:join)

try_require
```

* `lazy-require-sleep-1.rb`:

```ruby
puts "#{Thread.current} Running require"
sleep 1
puts "#{Thread.current} Require finished"
```

* Output:

```
$ ruby lazy-require.rb
 #<Thread:0x00007fc08b9081e0 lazy-require.rb:7 run> Going to require
 #<Thread:0x00007fc08b90be80 lazy-require.rb:7 run> Going to require
 #<Thread:0x00007fc08b9081e0 lazy-require.rb:7 run> Running require
 #<Thread:0x00007fc08b9081e0 lazy-require.rb:7 run> Require finished
 #<Thread:0x00007fc08b9081e0 lazy-require.rb:7 run> After require
 #<Thread:0x00007fc08b90be80 lazy-require.rb:7 run> After require
 #<Thread:0x00007fc08b85fa90 run> Going to require
 #<Thread:0x00007fc08b85fa90 run> After require
```

Notice how two threads tried to require, but only one actually ran the
code on the other file, while the other was blocked and only woke
up after the `require` finished.

Fixes #1306

lib/ddtrace/metrics.rb

@@ -41,7 +41,7 @@ def default_port
     end
 
     def default_statsd_client
-      require 'datadog/statsd' unless defined?(::Datadog::Statsd)
+      require 'datadog/statsd'
 
       # Create a StatsD client that points to the agent.
       Datadog::Statsd.new(default_hostname, default_port)