Add metric initializer spec

An empty file in one of the instrumented directories will cause the app
to fail to start when metrics are enabled. Metrics aren't enabled by
default in development or test.

We could handle the empty file case explicitly, but a file could still
not define the constant it is expected to, so instead run the
initializer manually in a spec and check that it succeeds.

spec/initializers/metrics_spec.rb

+require 'spec_helper'
+require_relative '../../config/initializers/metrics'
+
+describe 'instrument_classes', lib: true do
+  let(:config) { double(:config) }
+
+  before do
+    allow(config).to receive(:instrument_method)
+    allow(config).to receive(:instrument_methods)
+    allow(config).to receive(:instrument_instance_methods)
+  end
+
+  it 'can autoload and instrument all files' do
+    expect { instrument_classes(config) }.not_to raise_error
+  end
+end