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>

trace2.h

@@ -52,6 +52,7 @@ struct json_writer;
  * [] trace2_data*      -- emit region/thread/repo data messages.
  * [] trace2_printf*    -- legacy trace[1] messages.
  * [] trace2_timer*     -- stopwatch timers (messages are deferred).
+ * [] trace2_counter*   -- global counters (messages are deferred).
  */
 
 /*
@@ -528,6 +529,42 @@ enum trace2_timer_id {
 void trace2_timer_start(enum trace2_timer_id tid);
 void trace2_timer_stop(enum trace2_timer_id tid);
 
+/*
+ * Define the set of global counters.
+ *
+ * We can add more at any time, but they must be defined at compile
+ * time (to avoid the need to dynamically allocate and synchronize
+ * them between different threads).
+ *
+ * These must start at 0 and be contiguous (because we use them
+ * elsewhere as array indexes).
+ *
+ * Any values added to this enum be also be added to the
+ * `tr2_counter_metadata[]` in `trace2/tr2_tr2_ctr.c`.
+ */
+enum trace2_counter_id {
+	/*
+	 * Define two counters for testing.  See `t/helper/test-trace2.c`.
+	 * These can be used for ad hoc testing, but should not be used
+	 * for permanent analysis code.
+	 */
+	TRACE2_COUNTER_ID_TEST1 = 0, /* emits summary event only */
+	TRACE2_COUNTER_ID_TEST2,     /* emits summary and thread events */
+
+	/* Add additional counter definitions before here. */
+	TRACE2_NUMBER_OF_COUNTERS
+};
+
+/*
+ * Increase the named global counter by value.
+ *
+ * Note that this adds `value` to the current thread's partial sum for
+ * this counter (without locking) and that the complete sum is not
+ * available until all threads have exited, so it does not return the
+ * new value of the counter.
+ */
+void trace2_counter_add(enum trace2_counter_id cid, uint64_t value);
+
 /*
  * Optional platform-specific code to dump information about the
  * current and any parent process(es).  This is intended to allow