trace2: add global counter mechanism

Add global counters mechanism to Trace2.

The Trace2 counters mechanism adds the ability to create a set of
global counter variables and an API to increment them efficiently.
Counters can optionally report per-thread usage in addition to the sum
across all threads.

Counter events are emitted to the Trace2 logs when a thread exits and
at process exit.

Counters are an alternative to `data` and `data_json` events.

Counters are useful when you want to measure something across the life
of the process, when you don't want per-measurement events for
performance reasons, when the data does not fit conveniently within a
region, or when your control flow does not easily let you write the
final total.  For example, you might use this to report the number of
calls to unzip() or the number of de-delta steps during a checkout.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-trace2.txt

@@ -805,6 +805,37 @@ The "value" field may be an integer or a string.
 }
 ------------
 
+`"th_counter"`::
+	This event logs the value of a counter variable in a thread.
+	This event is generated when a thread exits for counters that
+	requested per-thread events.
++
+------------
+{
+	"event":"th_counter",
+	...
+	"category":"my_category",
+	"name":"my_counter",
+	"count":23
+}
+------------
+
+`"counter"`::
+	This event logs the value of a counter variable across all threads.
+	This event is generated when the process exits.  The total value
+	reported here is the sum across all threads.
++
+------------
+{
+	"event":"counter",
+	...
+	"category":"my_category",
+	"name":"my_counter",
+	"count":23
+}
+------------
+
+
 == Example Trace2 API Usage
 
 Here is a hypothetical usage of the Trace2 API showing the intended