Refactor mr counts data source + add mttm visualization

What does this MR do and why?

This MR refactors the merge request counts data source to provide a more flexible API that can support different metrics. It also adds a Mean Time To Merge (MTTM) visualization to the analytics dashboards.

The current implementation only supports merge request counts metrics, but our users need insights about merge request timing to better understand their development cycles. By adding MTTM visualization, teams can track how quickly code changes are being reviewed and merged, which is a key indicator of workflow efficiency.

Key Changes

Component	Change
Data Source	Refactored merge_request_counts.js to support multiple metrics
Visualization	Added new MTTM (Mean Time To Merge) visualization
Constants	Added metric type constants for better type safety
JSON Schema	Updated to support 365-day range for analytics dashboards
Bug Fix	Fixed humanizeDisplayUnit to properly handle "-" values

References

• Related to #523553 (closed)

Screenshots

No visual changes in this MR

How to set up and validate locally

1. Run the Jest tests for the refactored components:

   yarn jest components/data_sources/merge_request_counts_spec.js

MR acceptance checklist

• Tests added for new functionality
• Code follows GitLab style guidelines
• Changes maintain backward compatibility
• Performance impact considered