Fix bug in lambda support with service_name overrides

[basepi]

What does this pull request do?

The JIT metadata overrides in our AWS lambda support had the side effect of not allowing for a custom-configured service_name. This fixes that issue and adds a test.

Related issues

Closes #1486

[apmmachine]

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• Start Time: 2022-03-22T17:49:12.555+0000

• Duration: 21 min 26 sec

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Test	Results
Failed	0
Passed	4842
Skipped	3207
Total	8049

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[beniwohli]

Makes sense!