Fix stack trace highlight for PHP

Pick a first alternative language for Sentry stack trace highlight,
when Rouge can't decide which lexer to choose.

`StackTraceHighlightDecorator` now works with PHP and Hack languages

lib/gitlab/error_tracking/stack_trace_highlight_decorator.rb

@@ -28,7 +28,7 @@ module Gitlab
       end
 
       def highlight_entry_context(filename, context)
-        language = Rouge::Lexer.guess_by_filename(filename).tag
+        language = guess_language_by_filename(filename)
 
         context.map do |line_number, line_of_code|
           [
@@ -38,6 +38,12 @@ module Gitlab
           ]
         end
       end
+
+      def guess_language_by_filename(filename)
+        Rouge::Lexer.guess_by_filename(filename).tag
+      rescue Rouge::Guesser::Ambiguous => e
+        e.alternatives.min_by(&:tag)&.tag
+      end
     end
   end
 end

spec/factories/error_tracking/error_event.rb

@@ -35,6 +35,16 @@ FactoryBot.define do
             [8, "}\n"]
           ]
         },
+        {
+          'function' => 'print',
+          'lineNo' => 3,
+          'filename' => 'hello_world.php',
+          'context' => [
+            [1, "// PHP/Hack example\n"],
+            [2, "<?php\n"],
+            [3, "echo 'Hello, World!';\n"]
+          ]
+        },
         {
           'filename' => 'blank.txt'
         }

spec/lib/gitlab/error_tracking/stack_trace_highlight_decorator_spec.rb

@@ -48,6 +48,16 @@ describe Gitlab::ErrorTracking::StackTraceHighlightDecorator do
               [8, '<span id="LC1" class="line" lang="swift"><span class="p">}</span></span>']
             ]
           },
+          {
+            'function' => 'print',
+            'lineNo' => 3,
+            'filename' => 'hello_world.php',
+              'context' => [
+                [1, '<span id="LC1" class="line" lang="hack"><span class="c1">// PHP/Hack example</span></span>'],
+                [2, '<span id="LC1" class="line" lang="hack"><span class="cp">&lt;?php</span></span>'],
+                [3, '<span id="LC1" class="line" lang="hack"><span class="k">echo</span> <span class="s1">\'Hello, World!\'</span><span class="p">;</span></span>']
+            ]
+          },
           {
             'filename' => 'blank.txt'
           }