Please support the ability to set SCRAM-SHA-1 as the auth mechanism

[Stono]

Currently it appears the exporter will first try SCRAM-SHA-256, then SCRAM-SHA-1, which makes sense.
However as i primarily use atlas this results in us logging an auth failure first.
It's not a huge deal, but it'd be nice if i could do something like --mongodb.auth-mechanism=SCRAM-SHA-1 to remove that.

[github-actions]

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