sink.text.pretty: use bt_common_g_string_append_printf

sink.text.pretty: use bt_common_g_string_append_printf

g_string_append_printf() internally allocates a temporary buffer
through use of vasnprintf for each call. This clearly appears at
the top of perf report.

Use babeltrace's own bt_common_g_string_append_printf which operates
directly on the GString buffer, increasing its size to nearby next power
of two as needed.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I60b4dccfa19c4321eb5997233826cfabaf924ff2
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1503
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>