fix(metrics): store service name in defaultDimensions to avoid clearing it

[dreamorosi]

Description of your changes

As described in #1145, v1.4.0 highlighted a bug that causes the service name to be cleared from metrics after flushing.

The issue contains examples of the behavior and proposes a fix which has been implemented in this PR.

How to verify this change

See checks in PR.

Related issues, RFCs

Issue number: #1145

PR status

Is this ready for review?: YES
Is it a breaking change?: NO

Checklist

• My changes meet the tenets criteria
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in areas that should be flagged with a TODO, or hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding changes to the examples
• My changes generate no new warnings
• The code coverage hasn't decreased
• I have added tests that prove my change is effective and works
• New and existing unit tests pass locally and in Github Actions
• Any dependent changes have been merged and published in downstream module
• The PR title follows the conventional commit semantics

Breaking change checklist

N/A

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[dreamorosi]

E2E results (passing): https://github.com/awslabs/aws-lambda-powertools-typescript/actions/runs/3420360656

[dreamorosi]

This change (& the one a few lines below) is needed because we are comparing the JSON.stringif-ied version of these strings, so order counts.

I could have converted to object comparison but wanted to keep the diff small.