Reduce logging from mongodb exporter when unauthorized

As a follow up on https://perconadev.atlassian.net/browse/PMM-12989 , When mongodb commands (such as getDiagnosticData) fail on MongoDB exporter, we log the error and the corresponding reasons.

Since these commands are run on every scrape, they can be spammy and grow really large (default scrape is 5s).

Proposed Solution

Check if we can execute MongoDB commands on startup and log authorization errors on startup. That way, we can ignore such errors subsequently.