Mongo breadcrumb: Do not mask filter value nil

[johnnyshields]

Goal

In Mongo queries, the value nil is a special in that it can impact the behavior of sparse indexes. Rather than masking it with "?" we should indicate that it is nil. There is no risk of this causing data leakage to the logs.

Design

This is implemented in the filter masking function.

Changeset

Only changes the Mongo breadcrumb functionality.

This also includes the changes from:

• https://github.com/bugsnag/bugsnag-ruby/pull/797 b/c I cannot commit from my Windows machine otherwise.
• https://github.com/bugsnag/bugsnag-ruby/pull/798 to avoid conflicts
• https://github.com/bugsnag/bugsnag-ruby/pull/799 to avoid conflicts

Testing

Tested in RSpec and by TableCheck in production.

[mclack]

Hi @johnnyshields

Thanks for raising this. Along with the other PRs you have put forward, we will take a look at this when time allows.