metrics: Add a `reason` label to the HS error metrics.

This adds a `reason` label to the `hs_intro_rejected_intro_req_count` and
`hs_rdv_error_count` metrics introduced in #40755.

Metric look up and intialization is now more a bit more involved. This may be
fine for now, but it will become unwieldy if/when we add more labels (and as
such will need to be refactored).

Also, in the future, we may want to introduce finer grained `reason` labels.
For example, the `invalid_introduce2` label actually covers multiple types of
errors that can happen during the processing of an INTRODUCE2 cell (such as
cell parse errors, replays, decryption errors).

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

2 files changed, 23 insertions, 0 deletions

diff --git a/src/lib/metrics/metrics_store_entry.c b/src/lib/metrics/metrics_store_entry.c
index 482ec8d7d9..971d9379bd 100644
--- a/src/lib/metrics/metrics_store_entry.c
+++ b/src/lib/metrics/metrics_store_entry.c
@@ -127,3 +127,23 @@ metrics_store_entry_has_label(const metrics_store_entry_t *entry,
 
   return smartlist_contains_string(entry->labels, label);
 }
+
+/** Return the first entry that has the given label, or NULL if none
+ * of the entries have the label. */
+metrics_store_entry_t *
+metrics_store_find_entry_with_label(const smartlist_t *entries,
+                                    const char *label)
+{
+  tor_assert(entries);
+  tor_assert(label);
+
+  SMARTLIST_FOREACH_BEGIN(entries, metrics_store_entry_t *, entry) {
+    tor_assert(entry);
+
+    if (smartlist_contains_string(entry->labels, label)) {
+      return entry;
+    }
+  } SMARTLIST_FOREACH_END(entry);
+
+  return NULL;
+}
diff --git a/src/lib/metrics/metrics_store_entry.h b/src/lib/metrics/metrics_store_entry.h
index e4dc7a8b9a..0e09e099fe 100644
--- a/src/lib/metrics/metrics_store_entry.h
+++ b/src/lib/metrics/metrics_store_entry.h
@@ -12,6 +12,7 @@
 #include "lib/cc/torint.h"
 
 #include "lib/metrics/metrics_common.h"
+#include "lib/container/smartlist.h"
 
 #ifdef METRICS_STORE_ENTRY_PRIVATE
 
@@ -57,6 +58,8 @@ void metrics_store_entry_free_(metrics_store_entry_t *entry);
 int64_t metrics_store_entry_get_value(const metrics_store_entry_t *entry);
 bool metrics_store_entry_has_label(const metrics_store_entry_t *entry,
                                    const char *label);
+metrics_store_entry_t *metrics_store_find_entry_with_label(
+        const smartlist_t *entries, const char *label);
 
 /* Modifiers. */
 void metrics_store_entry_add_label(metrics_store_entry_t *entry,