Use hikari pool name as service name

[mwisnicki]

I have 2 postgresql connections and 2 redshift connections.

In order to differentiate them one can set DD_TRACE_DB_CLIENT_SPLIT_BY_INSTANCE but in my case I have database of the same name in postgresql and redshift which would create a conflict.

Another option is DD_TRACE_DB_CLIENT_SPLIT_BY_HOST but that requires more complicated service mappings and would be problematic if someone has 2 databases on the same server.

We already have to set unique pool names for Hikari so it would be great if dd could just pick them up as service names.