cql3: add tracing indexed aggregate queries
cql3: add tracing indexed aggregate queries

Commit 1c99ed6ced439bdf5a91e17689f4bc6bc7a6a281 added tracing logs
about the index chosen for the query, but aggregate queries have
a separate code path, which wasn't taken into account.
After this patch, tracing for aggregate queries also includes
this additional information.

Closes #10195

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla