Pass log level explicitly when building Gitlab::Logger

Note, the log level of Ruby's Logger defaults to DEBUG
https://ruby-doc.org/stdlib-2.6.6/libdoc/logger/rdoc/Logger.html
so we are not changing the log level but making it extra clear that
the choice of the log level is intentional and not configurable
for structured logging (yet).

lib/gitlab/logger.rb

@@ -32,7 +32,8 @@ module Gitlab
     end
 
     def self.build
-      Gitlab::SafeRequestStore[self.cache_key] ||= new(self.full_log_path)
+      Gitlab::SafeRequestStore[self.cache_key] ||=
+        new(self.full_log_path, level: ::Logger::DEBUG)
     end
 
     def self.full_log_path