PG-542: Performance improvement of pg_stat_monitor.

Performance related changes where some calculations are moved out
of the spinlock in the pgsm_update_entry function. This should
improve the performance a bit.

Also, moved the histogram calculation function to init. The update
function now only searches an array rather than recalculatiing the
histogram bucket timings.

Updated conditional statement to update parent query only when
required.

pg_stat_monitor.h

@@ -74,6 +74,7 @@
 
 #define JUMBLE_SIZE				1024	/* query serialization buffer size */
 
+#define HISTOGRAM_MAX_TIME		INT_MAX
 #define MAX_RESPONSE_BUCKET 50
 #define INVALID_BUCKET_ID	-1
 #define MAX_BUCKETS			10
@@ -422,6 +423,8 @@ typedef struct pgsmSharedState
 								/* context to store stats in local
 								 * memory until they are pushed to shared hash
 								 */
+	int			resp_calls[MAX_RESPONSE_BUCKET + 2];	/* execution time's in
+													 	 * msec; including 2 outlier buckets */
 	bool pgsm_oom;
 } pgsmSharedState;
 
@@ -510,6 +513,13 @@ static const struct config_enum_entry track_options[] =
 	{NULL, 0, false}
 };
 
+typedef enum
+{
+	HISTOGRAM_START,
+	HISTOGRAM_END,
+	HISTOGRAM_COUNT
+} HistogramTimingType;
+
 #define PGSM_MAX get_conf(0)->guc_variable
 #define PGSM_QUERY_MAX_LEN get_conf(1)->guc_variable
 #define PGSM_TRACK_UTILITY get_conf(2)->guc_variable