pre-define capacities for rollups and TLFs

[lizthegrey]

Which problem is this PR solving?

• Similar to pre-define map capacities libhoney-go#197, profiling shows we spend a lot of time in github.com/honeycombio/beeline-go/trace.(*Span).send generating the map.

Short description of the changes

• define the map capacity in advance before returning it.

a subsequent follow-up might be allowing passing a map in its entirety to github.com/honeycombio/libhoney-go.(*Event).AddField so that the map entries do not need to be added one at a time when merging the maps together.

Or, better yet, golang/go#56182 could prevent this problem by giving us a more sensible API for mass additions to a map.

[MikeGoldsmith]

Thanks @lizthegrey 👍🏻