Add new prometheus counter for corrupted build logs

This commit adds a new metric we will increment once we detect a
corrupted build log. Corrupted build log is detected when checking CRC32
checksum yields an invalid result and the build size differs from the
size reported by a runner.

lib/gitlab/ci/trace/checksum.rb

@@ -64,6 +64,12 @@ module Gitlab
           end
         end
 
+        def trace_size
+          trace_chunks.reduce(0) do |total, chunk|
+            total + chunk_size(chunk)
+          end
+        end
+
         def chunks_count
           trace_chunks.to_a.size
         end

lib/gitlab/ci/trace/metrics.rb

@@ -18,7 +18,8 @@ module Gitlab
           :conflict,  # runner has sent unrecognized build state details
           :locked,    # build trace has been locked by a different mechanism
           :stalled,   # failed to migrate chunk due to a worker duplication
-          :invalid    # malformed build trace has been detected using CRC32
+          :invalid,   # invalid build trace has been detected using CRC32
+          :corrupted  # malformed trace found after comparing CRC32 and size
         ].freeze
 
         def increment_trace_operation(operation: :unknown)

spec/lib/gitlab/ci/trace/checksum_spec.rb

@@ -113,6 +113,18 @@ RSpec.describe Gitlab::Ci::Trace::Checksum do
     end
   end
 
+  describe '#trace_size' do
+    before do
+      create_chunk(index: 0, data: 'a' * 128.kilobytes)
+      create_chunk(index: 1, data: 'b' * 128.kilobytes)
+      create_chunk(index: 2, data: 'abcdefg-ü')
+    end
+
+    it 'returns total trace size in bytes' do
+      expect(subject.trace_size).to eq 262154
+    end
+  end
+
   def create_chunk(index:, data:)
     create(:ci_build_trace_chunk, :persisted, build: build,
                                               chunk_index: index,