Add additional healthcheck for MongoDB
Add additional healthcheck for MongoDB

This ensures we correctly report this app as unhealthy if it cannot
connect to its database, which can happen on single instances [1].
While this may be incidentally covered by Smoke tests, it's clearer
if we call it out explicitly.

[1]: https://docs.google.com/document/d/1q0dmaRaK1pnoKK_EGikdstUOtBEiaCaswMYS9ITFbL0/edit

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