Fix: lttng-sessiond: output stream metadata before events

Fix: lttng-sessiond: output stream metadata before events

When trying the `doc/examples/demo` example from the lttng-ust
repository, the resulting trace's metadata lists some events before the
corresponding stream declaration. Here's an excerpt:

    event {
            name = "lttng_ust_statedump:end";
        id = 5;
        stream_id = 0;
        loglevel = 13;
        fields := struct {
            };
    };

    stream {
            id = 0;
            event.header := struct event_header_large;
            packet.context := struct packet_context;
    };

I don't know if this is allowed in CTF 1, but it won't be in CTF 2 (an
event record class fragment must come after its parent data stream class
fragment). In any case, I think it makes more sense to have the stream
first.

What I can see is that the ust_metadata_event_statedump function (which
emits the `event` declarations) is called for the statedump events
before the ust_metadata_channel_statedump function (which emits the
`stream` declaration) is called. A simple fix, as implemented in this
patch, is to delay emitting the event declarations until the stream
declaration has been emitted. To do so, return early in
ust_metadata_event_statedump if the `chan->metadata_dumped` flag is not
set. Then, when emitting the stream declaration, in
ust_metadata_event_statedump, emit any existing event, which have
presumably been skipped before hand.

It's possible that ust_metadata_event_statedump getting called before
ust_metadata_channel_statedump is a symptom of some more fundamental
problem over which this patch only papers over, but I don't know enough
about this to be able to tell.

I couldn't think of an appropriate test to write for this. However, once
we generate CTF2, such a bug would likely be caught by trace readers
rejecting the invlid metadata. So if we were to re-introduce this bug,
we would notice.

Change-Id: I6e3158c801fcc01b318618890704d19b3230e7a5
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>