Sometimes we want to fill out attributes for logging without inlining them into LOGV2 macro.

Example: https://github.com/mongodb/mongo/blob/7b9c390b626f344bda268da9fcf048e0dc003921/src/mongo/db/op_debug.cpp#L198

When we append a Date_t attribute, it is captured as CustomAttributeValue.

However, when CustomAttributeValue is created, it captures the value to be logged via reference: 

https://github.com/mongodb/mongo/blob/7b9c390b626f344bda268da9fcf048e0dc003921/src/mongo/logv2/attribute_storage.h#L335

So if we want to log a Date_t object, we must ensure that the same instance of Date_t is alive between adding it to DynamicAttributes and actually logging those DynamicAttributes via LOGV2 macro.

We should consider capturing small trivially-copyable objects by-value in CustomAttributeValue.