Fix: ctf: wrongfully requiring CTF metadata signature for every section

Fix: ctf: wrongfully requiring CTF metadata signature for every section

Issue
=====
When testing the special live tracing session case of sending the
metadata in multiple batches, we witnessed the CTF metadata decoder
returning an error if the "/* CTF 1.8" signature is omitted from the
metadata sections following the first one.

Here is the output of Babeltrace when this error occurs:
07-22 12:18:38.518 539222 539222 W PLUGIN/CTF/META/DECODER ctf_metadata_decoder_append_content@decoder.c:242 [lttng-live] Missing "/* CTF major.minor" signature in plain text metadata file stream: mdec
-addr=0x2398ec0
07-22 12:18:38.518 539222 539222 E PLUGIN/CTF/META/DECODER ctf_metadata_decoder_append_content@decoder.c:251 [lttng-live] Invalid metadata version found in plain text signature: version=571641920.0, md
ec-addr=0x2398ec0

We expect the decoder to only require this signature when decoding the
first section of plain-text metadata.

Solution
========
Only check for the signature when processing the first metadata section.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I3153abf58ade5f5b02362dbedde382f398f5ee4e
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3807