Reduce allocations in DataPointSerializer

[gfoidl]

DataPointSerializer can use pipelines / spans in order to reduce some allocations.
Or move this serializer to a separate package, as it should not be part of the "core-functionality".

[gfoidl]

DataPointServializer is a bad name anyway. This type shouldn't exist at all.
Due it's used for tests, I should make this type internal and / or move it to the test-project and don't ship with the product.

There a better ways to serialze the datapoints.