add server_name to transaction data

[nrxus]

Currently transactions do not include a server_name which makes it harder to know where a transaction occurred.

According to the transaction docs:

Transactions are Events enriched with Span data

And the events docs include server_name so I don't see any reason not to include it.

Perhaps in the future it might make more sense to have a common struct between the two that gets flattened (through serde) such that when Event gets new fields that may be useful in a Transaction then it'll already be there (as long as the function to enrich that hypothetical shared struct is used in both places). This, however, is outside of the scope of what I need at this time so I only did the smallest thing I needed.