net: prometheus: summary: Add set function to summary metric

Add prometheus_summary_observe_set() function to summary metric.

Signed-off-by: Jukka Rissanen <[email protected]>

Author: jukkar

include/zephyr/net/prometheus/summary.h

@@ -96,6 +96,20 @@ struct prometheus_summary {
  */
 int prometheus_summary_observe(struct prometheus_summary *summary, double value);
 
+/**
+ * @brief Set the summary value to specific value.
+ * The new value must be higher than the current value. This function can be used
+ * if we cannot add individual increments to the summary but need to periodically
+ * update the counter value. This function will add the difference between the
+ * new value and the old value to the summary fields.
+ * @param summary Pointer to the summary metric to increment.
+ * @param value New value of the summary.
+ * @param count New counter value of the summary.
+ * @return 0 on success, negative errno on error.
+ */
+int prometheus_summary_observe_set(struct prometheus_summary *summary,
+				   double value, unsigned long count);
+
 /**
  * @}
  */

subsys/net/lib/prometheus/summary.c

@@ -28,3 +28,31 @@ int prometheus_summary_observe(struct prometheus_summary *summary, double value)
 
 	return 0;
 }
+
+int prometheus_summary_observe_set(struct prometheus_summary *summary,
+				   double value, unsigned long count)
+{
+	unsigned long old_count;
+	double old_sum;
+
+	if (summary == NULL) {
+		return -EINVAL;
+	}
+
+	if (value == summary->sum && count == summary->count) {
+		return 0;
+	}
+
+	old_count = summary->count;
+	if (count < old_count) {
+		LOG_DBG("Cannot set summary count to a lower value");
+		return -EINVAL;
+	}
+
+	summary->count += (count - old_count);
+
+	old_sum = summary->sum;
+	summary->sum += (value - old_sum);
+
+	return 0;
+}