lib: New metrics library

Used to provide an interface to create metrics store and update the entries.

Related to #40063

Signed-off-by: David Goulet <dgoulet@torproject.org>

1 files changed, 56 insertions, 0 deletions

diff --git a/src/lib/metrics/prometheus.c b/src/lib/metrics/prometheus.c
new file mode 100644
index 0000000000..c2b54e436f
--- /dev/null
+++ b/src/lib/metrics/prometheus.c
@@ -0,0 +1,56 @@
+/* Copyright (c) 2020, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+/**
+ * @file prometheus.c
+ * @brief Metrics format driver for Prometheus data model.
+ **/
+
+#define METRICS_STORE_ENTRY_PRIVATE
+
+#include "orconfig.h"
+
+#include "lib/container/smartlist.h"
+#include "lib/log/util_bug.h"
+#include "lib/malloc/malloc.h"
+#include "lib/string/printf.h"
+
+#include "lib/metrics/prometheus.h"
+
+/** Return a static buffer containing all the labels properly formatted
+ * for the output as a string.
+ *
+ * Subsequent calls to this invalidates the previous result. */
+static const char *
+format_labels(smartlist_t *labels)
+{
+  static char buf[1024];
+  char *line = NULL;
+
+  if (smartlist_len(labels) == 0) {
+    buf[0] = '\0';
+    goto end;
+  }
+
+  line = smartlist_join_strings(labels, ",", 0, NULL);
+  tor_snprintf(buf, sizeof(buf), "{%s}", line);
+
+ end:
+  tor_free(line);
+  return buf;
+}
+
+/** Format the given entry in to the buffer data. */
+void
+prometheus_format_store_entry(const metrics_store_entry_t *entry, buf_t *data)
+{
+  tor_assert(entry);
+  tor_assert(data);
+
+  buf_add_printf(data, "# HELP %s %s\n", entry->name, entry->help);
+  buf_add_printf(data, "# TYPE %s %s\n", entry->name,
+                 metrics_type_to_str(entry->type));
+  buf_add_printf(data, "%s%s %" PRIi64 "\n", entry->name,
+                 format_labels(entry->labels),
+                 metrics_store_entry_get_value(entry));
+}