tr2: shows scope unconditionally in addition to key-value pair

When we specify GIT_TRACE2_CONFIG_PARAMS or trace2.configparams,
trace2 will prints "interesting" config values to log. Sometimes,
when a config set in multiple scope files, the following output
looks like (the irrelevant fields are omitted here as "..."):

...| def_param    |  ...  | core.multipackindex:false
...| def_param    |  ...  | core.multipackindex:false
...| def_param    |  ...  | core.multipackindex:false

As the log shows, even each config in different scope is dumped, but
we don't know which scope it comes from. Therefore, it's better to
add the scope names as well to make them be more recognizable. For
example, when execute:

    $ GIT_TRACE2_PERF=1 \
    > GIT_TRACE2_CONFIG_PARAMS=core.multipackIndex \
    > git rev-list --test-bitmap HEAD"

The following is the ouput (the irrelevant fields are omitted here
as "..."):

Format normal:
... git.c:461 ... def_param scope:system core.multipackindex=false
... git.c:461 ... def_param scope:global core.multipackindex=false
... git.c:461 ... def_param scope:local core.multipackindex=false

Format perf:

... | def_param    | ... | scope:system | core.multipackindex:false
... | def_param    | ... | scope:global | core.multipackindex:false
... | def_param    | ... | scope:local  | core.multipackindex:false

Format event:

{"event":"def_param", ... ,"scope":"system","param":"core.multipackindex","value":"false"}
{"event":"def_param", ... ,"scope":"global","param":"core.multipackindex","value":"false"}
{"event":"def_param", ... ,"scope":"local","param":"core.multipackindex","value":"false"}

Signed-off-by: Teng Long <dyroneteng@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-trace2.txt

@@ -717,6 +717,7 @@ The "exec_id" field is a command-unique id and is only useful if the
 {
 	"event":"def_param",
 	...
+	"scope":"global",
 	"param":"core.abbrev",
 	"value":"7"
 }
@@ -1216,7 +1217,13 @@ We can optionally emit configuration events, see
 it.
 +
 ----------------
-$ git config color.ui auto
+$ git config --system color.ui never
+$ git config --global color.ui always
+$ git config --local color.ui auto
+$ git config --list --show-scope | grep 'color.ui'
+system  color.ui=never
+global  color.ui=always
+local   color.ui=auto
 ----------------
 +
 Then, mark the config `color.ui` as "interesting" config with
@@ -1232,11 +1239,13 @@ $ cat ~/log.perf
 d0 | main                     | version      |     |           |           |              | ...
 d0 | main                     | start        |     |  0.001642 |           |              | /usr/local/bin/git version
 d0 | main                     | cmd_name     |     |           |           |              | version (version)
-d0 | main                     | def_param    |     |           |           |              | color.ui:auto
+d0 | main                     | def_param    |     |           |           | scope:system | color.ui:never
+d0 | main                     | def_param    |     |           |           | scope:global | color.ui:always
+d0 | main                     | def_param    |     |           |           | scope:local  | color.ui:auto
 d0 | main                     | data         | r0  |  0.002100 |  0.002100 | fsync        | fsync/writeout-only:0
 d0 | main                     | data         | r0  |  0.002126 |  0.002126 | fsync        | fsync/hardware-flush:0
-d0 | main                     | exit         |     |  0.002142 |           |              | code:0
+d0 | main                     | exit         |     |  0.000470 |           |              | code:0
-d0 | main                     | atexit       |     |  0.002161 |           |              | code:0
+d0 | main                     | atexit       |     |  0.000477 |           |              | code:0
 ----------------
 == Future Work
 

trace2/tr2_tgt_event.c

@@ -479,9 +479,12 @@ static void fn_param_fl(const char *file, int line, const char *param,
 {
 	const char *event_name = "def_param";
 	struct json_writer jw = JSON_WRITER_INIT;
+	enum config_scope scope = current_config_scope();
+	const char *scope_name = config_scope_name(scope);
 
 	jw_object_begin(&jw, 0);
 	event_fmt_prepare(event_name, file, line, NULL, &jw);
+	jw_object_string(&jw, "scope", scope_name);
 	jw_object_string(&jw, "param", param);
 	jw_object_string(&jw, "value", value);
 	jw_end(&jw);

trace2/tr2_tgt_normal.c

@@ -298,8 +298,11 @@ static void fn_param_fl(const char *file, int line, const char *param,
 			const char *value)
 {
 	struct strbuf buf_payload = STRBUF_INIT;
+	enum config_scope scope = current_config_scope();
+	const char *scope_name = config_scope_name(scope);
 
-	strbuf_addf(&buf_payload, "def_param %s=%s", param, value);
+	strbuf_addf(&buf_payload, "def_param scope:%s %s=%s", scope_name, param,
+		    value);
 	normal_io_write_fl(file, line, &buf_payload);
 	strbuf_release(&buf_payload);
 }

trace2/tr2_tgt_perf.c

@@ -441,12 +441,17 @@ static void fn_param_fl(const char *file, int line, const char *param,
 {
 	const char *event_name = "def_param";
 	struct strbuf buf_payload = STRBUF_INIT;
+	struct strbuf scope_payload = STRBUF_INIT;
+	enum config_scope scope = current_config_scope();
+	const char *scope_name = config_scope_name(scope);
 
 	strbuf_addf(&buf_payload, "%s:%s", param, value);
+	strbuf_addf(&scope_payload, "%s:%s", "scope", scope_name);
 
-	perf_io_write_fl(file, line, event_name, NULL, NULL, NULL, NULL,
+	perf_io_write_fl(file, line, event_name, NULL, NULL, NULL,
-			 &buf_payload);
+			 scope_payload.buf, &buf_payload);
 	strbuf_release(&buf_payload);
+	strbuf_release(&scope_payload);
 }
 
 static void fn_repo_fl(const char *file, int line,