fix: impact metric serialization and overload

[sjaanus]

Fix enum serialization and add overload.

1. Previously the enums were going to Unleash in caps and it failed to parse it. It must be small case. Added serialization annotation to solve it.
2. Noticed we had one missing overload for histogram. Added it.

[coveralls]

Pull Request Test Coverage Report for Build 20570588209

Details

• 8 of 8 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 79.428%

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Totals
Change from base Build 20457872467:	0.2%
Covered Lines:	3210
Relevant Lines:	3893

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💛 - Coveralls