Freeze time in metrics value expectation

What does this MR do and why?

See: #362591 (closed)

This fixes a master:broken issue with an unreliable spec.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

spec/support/gitlab/usage/metrics_instrumentation_shared_examples.rb

@@ -5,6 +5,10 @@
   let(:options) { params[:options] }
   let(:metric) { described_class.new(time_frame: time_frame, options: options) }
 
+  around do |example|
+    freeze_time { example.run }
+  end
+
   before do
     if described_class.respond_to?(:relation) && described_class.relation.respond_to?(:connection)
       allow(described_class.relation.connection).to receive(:transaction_open?).and_return(false)