Use sentinel value for truncated param logging

Our Elasticsearch mapping on GitLab.com requires params to be an array
of hashes, each hash with a `key` and `value` key. This does not apply
to our Sidekiq arguments logging, as Sidekiq arguments can be of
different types anyway, and do not have keys.

config/initializers/lograge.rb

@@ -28,7 +28,7 @@ unless Gitlab::Runtime.sidekiq?
 
       payload = {
         time: Time.now.utc.iso8601(3),
-        params: Gitlab::Utils::LogLimitedArray.log_limited_array(params),
+        params: Gitlab::Utils::LogLimitedArray.log_limited_array(params, sentinel: { key: 'truncated', value: '...' }),
         remote_ip: event.payload[:remote_ip],
         user_id: event.payload[:user_id],
         username: event.payload[:username],

lib/gitlab/grape_logging/formatters/lograge_with_timestamp.rb

@@ -30,7 +30,8 @@ module Gitlab
               .each_pair
               .map { |k, v| { key: k, value: utf8_encode_values(v) } }
 
-          Gitlab::Utils::LogLimitedArray.log_limited_array(params_array)
+          Gitlab::Utils::LogLimitedArray.log_limited_array(params_array,
+                                                           sentinel: { key: 'truncated', value: '...' })
         end
 
         def utf8_encode_values(data)

lib/gitlab/utils/log_limited_array.rb

@@ -6,9 +6,9 @@ module Gitlab
       MAXIMUM_ARRAY_LENGTH = 10.kilobytes
 
       # Prepare an array for logging by limiting its JSON representation
-      # to around 10 kilobytes. Once we hit the limit, add "..." as the
-      # last item in the returned array.
-      def self.log_limited_array(array)
+      # to around 10 kilobytes. Once we hit the limit, add the sentinel
+      # value as the last item in the returned array.
+      def self.log_limited_array(array, sentinel: '...')
         return [] unless array.is_a?(Array)
 
         total_length = 0
@@ -18,7 +18,7 @@ module Gitlab
           total_length <= MAXIMUM_ARRAY_LENGTH
         end
 
-        limited_array.push('...') if total_length > MAXIMUM_ARRAY_LENGTH
+        limited_array.push(sentinel) if total_length > MAXIMUM_ARRAY_LENGTH
 
         limited_array
       end

spec/initializers/lograge_spec.rb

@@ -17,7 +17,7 @@ describe 'lograge', type: :request do
   end
 
   let(:limited_params) do
-    large_params.slice(:a, :b).map { |k, v| { key: k.to_s, value: v } } + ['...']
+    large_params.slice(:a, :b).map { |k, v| { key: k.to_s, value: v } } + [{ key: 'truncated', value: '...' }]
   end
 
   context 'for API requests' do

spec/lib/gitlab/utils/log_limited_array_spec.rb

@@ -18,15 +18,29 @@ describe Gitlab::Utils::LogLimitedArray do
       end
 
       context 'when the array exceeds the limit' do
-        it 'replaces arguments after the limit with an ellipsis string' do
+        let(:long_array) do
           half_limit = described_class::MAXIMUM_ARRAY_LENGTH / 2
-          long_array = ['a' * half_limit, 'b' * half_limit, 'c']
 
+          ['a' * half_limit, 'b' * half_limit, 'c']
+        end
+
+        context 'when no sentinel value is passed' do
+          it 'replaces arguments after the limit with an ellipsis string' do
             expect(described_class.log_limited_array(long_array))
               .to eq(long_array.take(1) + ['...'])
           end
         end
 
+        context 'when a sentinel value is passed' do
+          it 'replaces arguments after the limit with the sentinel' do
+            sentinel = { truncated: true }
+
+            expect(described_class.log_limited_array(long_array, sentinel: sentinel))
+              .to eq(long_array.take(1) + [sentinel])
+          end
+        end
+      end
+
       context 'when the array contains arrays and hashes' do
         it 'calculates the size based on the JSON representation' do
           long_array = [