metrics: Add support for histograms.

This will enable us to add e.g. circuit build metrics (#40717).

Signed-off-by: Gabriela Moldovan <gabi@torproject.org>

1 files changed, 23 insertions, 0 deletions

diff --git a/src/lib/metrics/metrics_common.h b/src/lib/metrics/metrics_common.h
index 3644ad3d50..6c8e8bdbdc 100644
--- a/src/lib/metrics/metrics_common.h
+++ b/src/lib/metrics/metrics_common.h
@@ -10,6 +10,7 @@
 #define TOR_LIB_METRICS_METRICS_COMMON_H
 
 #include "lib/cc/torint.h"
+#include "lib/container/smartlist.h"
 
 /** Helper macro that must be used to construct the right namespaced metrics
  * name. A name is a string so stringify the result. */
@@ -28,8 +29,18 @@ typedef enum {
   METRICS_TYPE_COUNTER,
   /* Can go up or down. */
   METRICS_TYPE_GAUGE,
+  /* Cumulative counters for multiple observation buckets. */
+  METRICS_TYPE_HISTOGRAM,
 } metrics_type_t;
 
+typedef struct metrics_histogram_bucket_t {
+  /* The value of the counter of this bucket. */
+  uint64_t value;
+  /* Technically, this should be a floating point value, but in practice, we
+   * can make do with integer buckets. */
+  int64_t bucket;
+} metrics_histogram_bucket_t;
+
 /** Metric counter object (METRICS_TYPE_COUNTER). */
 typedef struct metrics_counter_t {
   uint64_t value;
@@ -40,6 +51,18 @@ typedef struct metrics_gauge_t {
   int64_t value;
 } metrics_gauge_t;
 
+/** Metric histogram object (METRICS_TYPE_HISTOGRAM). */
+typedef struct metrics_histogram_t {
+  /* The observation buckets. */
+  metrics_histogram_bucket_t *buckets;
+  /* The number of observation buckets. */
+  size_t bucket_count;
+  /* The sum of all observations */
+  int64_t sum;
+  /* The total number of observations */
+  uint64_t count;
+} metrics_histogram_t;
+
 const char *metrics_type_to_str(const metrics_type_t type);
 
 /* Helpers. */