trace2: increment event format version

In 64bc752 (trace2: add trace2_child_ready() to report on background
children, 2021-09-20), we added a new "child_ready" event. In
Documentation/technical/api-trace2.txt, we promise that adding a new
event type will result in incrementing the trace2 event format version
number, but this was not done. Correct this in code & docs.

Signed-off-by: Josh Steadmon <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: steadmon

Documentation/technical/api-trace2.txt

@@ -128,7 +128,7 @@ yields
 
 ------------
 $ cat ~/log.event
-{"event":"version","sid":"sid":"20190408T191610.507018Z-H9b68c35f-P000059a8","thread":"main","time":"2019-01-16T17:28:42.620713Z","file":"common-main.c","line":38,"evt":"2","exe":"2.20.1.155.g426c96fcdb"}
+{"event":"version","sid":"sid":"20190408T191610.507018Z-H9b68c35f-P000059a8","thread":"main","time":"2019-01-16T17:28:42.620713Z","file":"common-main.c","line":38,"evt":"3","exe":"2.20.1.155.g426c96fcdb"}
 {"event":"start","sid":"20190408T191610.507018Z-H9b68c35f-P000059a8","thread":"main","time":"2019-01-16T17:28:42.621027Z","file":"common-main.c","line":39,"t_abs":0.001173,"argv":["git","version"]}
 {"event":"cmd_name","sid":"20190408T191610.507018Z-H9b68c35f-P000059a8","thread":"main","time":"2019-01-16T17:28:42.621122Z","file":"git.c","line":432,"name":"version","hierarchy":"version"}
 {"event":"exit","sid":"20190408T191610.507018Z-H9b68c35f-P000059a8","thread":"main","time":"2019-01-16T17:28:42.621236Z","file":"git.c","line":662,"t_abs":0.001227,"code":0}
@@ -391,7 +391,7 @@ only present on the "start" and "atexit" events.
 {
 	"event":"version",
 	...
-	"evt":"2",		       # EVENT format version
+	"evt":"3",		       # EVENT format version
 	"exe":"2.20.1.155.g426c96fcdb" # git version
 }
 ------------

trace2/tr2_tgt_event.c

@@ -20,7 +20,7 @@ static struct tr2_dst tr2dst_event = { TR2_SYSENV_EVENT, 0, 0, 0, 0 };
  * a new field to an existing event, do not require an increment to the EVENT
  * format version.
  */
-#define TR2_EVENT_VERSION "2"
+#define TR2_EVENT_VERSION "3"
 
 /*
  * Region nesting limit for messages written to the event target.