[mongo] mongodb operation sampling with DBM

[lu-zhengda]

What does this PR do?

This PR adds a new feature to support mongodb operation sampling when DBM is enabled. The change includes

• Samples mongodb operations with $currentOp aggregation pipeline. Sampled operation includes all users and idle sessions
• Collect operation explain plans for sampled operations
  This feature is gated by DBM and cannot be enabled without DBM.

Motivation

https://datadoghq.atlassian.net/browse/DBMON-3997

Additional Notes

Sampled Statements

Explain Plans

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Changelog entries must be created for modifications to shipped code
• 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

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