Add allocated objects count to our structured logs and distributed tracing

As of Rails 6.0 we're getting Ruby allocated objects count out of the box (https://blog.bigbinary.com/2019/07/23/rails-6-reports-object-allocations-made-while-rendering-view-templates.html).

It means we're probably able to add this data without much effort at our structured logs through Gitlab::InstrumentationHelper, giving further information for web calls.

Additionally, we can also make a change to labkit-ruby ActiveSupportSubscriber in order to add this data to our distributed tracing (which is currently only available locally for GDK).