[inspector] Display count in the header for objects with known size

[alexander-yakushev]

Similarly to how it was already done for arrays, Counted objects now also show their size upfront.

• [x] You've added tests to cover your change(s)
• [x] You've updated the changelog (if adding/changing user-visible functionality)

[bbatsov]

The code looks good to me, although it seems there's a small spacing mistake that results in failing CI.