JSON formatter does not render backtraces for multiple exceptions

[p-mongo]

Subject of the issue

Example result:

    {
      "id": ".\/spec\/mongo\/bulk_write_spec.rb[1:1:1:1:2:10:2:1]",
      "description": "does not apply the collation",
      "full_description": "Mongo::BulkWrite#execute when the bulk write is unordered behaves like an executable bulk write when the operations do not need to be split when providing multiple update ones when the write does not have a collation specified does not apply the collation",
      "status": "failed",
      "file_path": ".\/spec\/mongo\/bulk_write_spec.rb",
      "line_number": 1474,
      "run_time": 0.011933,
      "pending_message": null,
      "exception": {
        "class": "RSpec::Core::MultipleExceptionError",
        "message": "command delete requires authentication (13)\n\ncommand delete requires authentication (13)",
        "backtrace": null
      }
    },

The exceptions along with their backtraces should be rendered by the json formatter, as they are rendered by other formatters.

Your environment

• Ruby version: 2.5
• rspec-core version: 3.7.1