sink.text.pretty: do not printf field name strings

Use g_string_append rather than the printf counterpart to output field
name strings. This significantly improves pretty printing speed
(34s -> 29s on reference trace).

This is a performance improvement mainly because only strings are passed
to printf in this case.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I4df52308c61346a81e0b79b079d53be52084707f
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1504
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>