[DBM Doc] Note minimum Agent version for MongoDB query metrics

[datadog-datadog-prod-us1]

Confidence: DISCUSSION

Source PR: DataDog/integrations-core#23596 ("Mongo Query Metrics")

What changed

Adds a "Minimum Datadog Agent version for MongoDB query metrics: 7.78" entry to the "Before you begin" section of the self-hosted MongoDB DBM setup page.

Why

The source PR added a callout to the MongoDB integration README noting that some metrics (query metrics) require Datadog Agent v7.78 or later. The same version requirement is relevant for users setting up DBM for MongoDB and is currently missing from the DBM doc.

Notes for reviewers

• Placement in "Before you begin" mirrors how other DBM setup pages list pre-requisites; happy to move this elsewhere (e.g., into a "Data Collected" callout) if preferred.
• The wording is intentionally narrow ("for MongoDB query metrics") since other MongoDB DBM features (slow operations, samples, explain plans, replication state) are not gated on Agent v7.78.

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/pierreln-dd/dbm-doc-mongo-23596-v2/database_monitoring/setup_mongodb/selfhosted