ECKIT-658 reverted metrics_ inside MetricsCollector to create compati…

[danovaro]

Description

This fixes a recent change inside MetricsCollector: metrics_ used to be stored as "eckit::Value", containing a ValueMap.
The breaking change used directly a ValueMap, which is slightly more convenient for value manipulation.

This PR reverts this (metrics_ is back to eckit::Value type), since it was affecting the way Metrics are serialised

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

🌦️ >> Documentation << 🌦️
https://sites.ecmwf.int/docs/dev-section/eckit/pull-requests/PR-253

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 66.27%. Comparing base (842765c) to head (39ad1dc).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/eckit/runtime/Metrics.cc	80.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #253      +/-   ##
===========================================
+ Coverage    66.24%   66.27%   +0.02%     
===========================================
  Files         1130     1130              
  Lines        57955    57931      -24     
  Branches      4394     4390       -4     
===========================================
- Hits         38393    38391       -2     
+ Misses       19562    19540      -22     

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