Addition of data_stream fields causes allocations when data streams are disabled

[axw]

Since #4409 was merged, there has been an increase in the allocations in hey-apm benchmarks:

Making some changes locally, this appears to be entirely due to the additional data_stream.* fields. When data streams are disabled, we add these fields and then later remove them with a processor (allowing centralisation of that logic).

The impact is more significant than I expected, and we should consider changing how this works: only add the fields when data streams are enabled. This could be done by adding a new field to transform.Config.