chore: FIx Metrics - Add missing array to wrap metrics array

[hjgraca]

Please provide the issue number

Issue number: #662

Summary

Changes

Add missing array that wraps Dimensions array

Empty Dimensions

{
  "_aws": {
    "Timestamp": 1728503062129,
    "CloudWatchMetrics": [
      {
        "Namespace": "dotnet-powertools-test",
        "Metrics": [
          {
            "Name": "Metric Name",
            "Unit": "Count"
          }
        ],
        "Dimensions": [
          []
        ]
      }
    ]
  },
  "Metric Name": 1
}

Dimensions with values

{
  "_aws": {
    "Timestamp": 1728503151889,
    "CloudWatchMetrics": [
      {
        "Namespace": "ns",
        "Metrics": [
          {
            "Name": "ColdStart",
            "Unit": "Count"
          }
        ],
        "Dimensions": [
          [
            "FunctionName",
            "Service"
          ]
        ]
      }
    ]
  },
  "FunctionName": "My Function with context",
  "Service": "svc",
  "ColdStart": 1
}

User experience

Please share what the user experience looks like before and after this change

Checklist

Please leave checklist items unchecked if they do not apply to your change.

• Meets tenets criteria
• I have performed a self-review of this change
• Changes have been tested
• Changes are documented
• PR title follows conventional commit semantics

Is this a breaking change?

RFC issue number:

Checklist:

• Migration process documented
• Implement warnings (if it can live side by side)

Acknowledgment

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[codecov]

Codecov Report

Attention: Patch coverage is 78.57143% with 6 lines in your changes missing coverage. Please review.

Project coverage is 73.44%. Comparing base (5eb8338) to head (31dad3b).

Files with missing lines	Patch %	Lines
...aries/src/AWS.Lambda.Powertools.Metrics/Metrics.cs	77.77%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #666   +/-   ##
========================================
  Coverage    73.43%   73.44%           
========================================
  Files          196      196           
  Lines         8027     8043   +16     
  Branches       857      858    +1     
========================================
+ Hits          5895     5907   +12     
- Misses        1846     1849    +3     
- Partials       286      287    +1     

Flag	Coverage Δ
unittests	73.44% <78.57%> (+<0.01%)	⬆️

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[sonarcloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud