PMM-13910 Fix hottest collections issue

[igroene]

PMM-13910

This pull request updates the MongoDB dashboards and their documentation to improve clarity, accuracy, and consistency in how hot collections and operation rates are displayed. The most important changes include standardizing units to "ops/sec", improving query expressions for stability, making collection names more visible in visualizations, and updating documentation to match the dashboard behavior.

Dashboard Query and Visualization Improvements:

• Standardized units for operations per second to "ops/sec" or "ops/s" across all relevant panels in MongoDB_Cluster_Summary.json, MongoDB_ReplSet_Summary.json, and MongoDB_Collections_Overview.json for clarity and consistency. [1] [2] [3] [4] [5] [6] [7]
• Refined PromQL expressions to use stable rate windows (e.g., rate(...[$__rate_interval])) for more accurate and reliable calculation of top hot collections by read and write, and simplified query logic for better maintainability. [1] [2] [3] [4] [5] [6] [7]
• Changed namePlacement from "auto" to "left" in bar gauge visualizations to ensure collection names are always visible, especially for long names. [1] [2] [3] [4] [5] [6] [7]

Documentation Updates:

• Updated descriptions in the dashboard documentation to clarify that hot collection panels use a stable rate window and improved bar gauge label visibility, reflecting the dashboard changes. [1] [2] [3]
• Minor improvements to documentation for clarity and consistency, such as capitalization and section formatting. [1] [2]

These changes collectively enhance the accuracy of displayed metrics, the usability of the dashboards, and the clarity of the accompanying documentation.