Enable MongoDB benchmarks in Github Actions

[ecton]

Currently, GitHub Actions doesn't include MongoDB in the benchmark suite. We need to launch MongoDB with a replication set, which requires customizing the command line arguments -- a feature GitHub Actions doesn't support on service containers.

The alternate is to use a 3rd party action or create our own service image.

For now, this isn't causing any issues, but it does mean that the MongoDB benchmark isn't being tested or included in code coverage.