[mongo] fix error CommandCursor is not subscriptable

[lu-zhengda]

What does this PR do?

This PR fixes error CommandCursor is not subscriptable in replication oplog size and oplog window collection.

Motivation

https://datadoghq.atlassian.net/browse/DBMON-4927

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.76%. Comparing base (3585433) to head (cd7b604).
Report is 11 commits behind head on master.

Additional details and impacted files

Flag	Coverage Δ
activemq	?
cassandra	?
hive	?
hivemq	?
ignite	?
jboss_wildfly	?
kafka	?
mongo	92.86% <100.00%> (-0.94%)	⬇️
presto	?
solr	?

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