Add check for unknown operation

lib/generators/gitlab/usage_metric/usage_metric_generator.rb

@@ -38,7 +38,7 @@ module Gitlab
     def validate!
       raise ArgumentError, "Type is required, valid options are #{ALLOWED_SUPERCLASSES.keys.join(', ')}" unless type.present?
       raise ArgumentError, "Unknown type '#{type}', valid options are #{ALLOWED_SUPERCLASSES.keys.join(', ')}" if metric_superclass.nil?
-      raise ArgumentError, "Unknown operation '#{operation}' valid operations are #{ALLOWED_OPERATIONS.join(', ')}" if type == 'database' && operation.nil?
+      raise ArgumentError, "Unknown operation '#{operation}' valid operations are #{ALLOWED_OPERATIONS.join(', ')}" if type == 'database' && !ALLOWED_OPERATIONS.include?(operation)
     end
 
     def ee?

spec/lib/generators/gitlab/usage_metric_generator_spec.rb

@@ -86,5 +86,13 @@ RSpec.describe Gitlab::UsageMetricGenerator, :silence_stdout do
         expect { described_class.new(args, options).invoke_all }.to raise_error(ArgumentError, /Unknown operation ''/)
       end
     end
+
+    context 'with wrong operation for database metric' do
+      let(:options) { { 'type' => 'database', 'operation' => 'sleep' } }
+
+      it 'raises an ArgumentError' do
+        expect { described_class.new(args, options).invoke_all }.to raise_error(ArgumentError, /Unknown operation 'sleep'/)
+      end
+    end
   end
 end