Enhance library/cpp/monlib exception verbosity for debugging

Этот эксепшн ронял YDB RU PRESTABLE минимум дважды:
\- 2025-09-10 <https://nda.ya.ru/t/VerKgHXk7VTLyV>
\- 2026-02-19 <https://nda.ya.ru/t/uhTY2yqK7UtBen>

Патч призван упростить отладку падений и поиски руткоза
commit_hash:72d859bae3ab265a2807e566c763bfb70a92a61a

1 files changed, 31 insertions, 0 deletions

diff --git a/library/cpp/monlib/dynamic_counters/encode_ut.cpp b/library/cpp/monlib/dynamic_counters/encode_ut.cpp
index 52d77b6b413..b1ef7df93a8 100644
--- a/library/cpp/monlib/dynamic_counters/encode_ut.cpp
+++ b/library/cpp/monlib/dynamic_counters/encode_ut.cpp
@@ -8,6 +8,7 @@
 #include <library/cpp/testing/unittest/registar.h>
 
 #include <util/generic/buffer.h>
+#include <util/generic/yexception.h>
 #include <util/stream/buffer.h>
 
 namespace NMonitoring {
@@ -221,6 +222,36 @@ namespace NMonitoring {
 
             AssertResult(samples);
         }
+
+        Y_UNIT_TEST(InvalidUtf8) {
+            TDynamicCounterPtr rootGroup1(new TDynamicCounters());
+            rootGroup1->GetNamedCounter("invalid\x80Utf8Name", "validUtf8");
+
+            {
+                TString result;
+                TStringOutput out(result);
+                auto encoder = CreateEncoder(&out, EFormat::JSON);
+                UNIT_ASSERT_EXCEPTION_CONTAINS(
+                    rootGroup1->Accept(TString(), TString(), *encoder),
+                    yexception,
+                    "label name is not valid UTF-8 string: 'invalid\\x80Utf8Name'"
+                );
+            }
+
+            TDynamicCounterPtr rootGroup2(new TDynamicCounters());
+            rootGroup2->GetNamedCounter("validUtf8Name", "invalid\x80Utf8");
+
+            {
+                TString result;
+                TStringOutput out(result);
+                auto encoder = CreateEncoder(&out, EFormat::JSON);
+                UNIT_ASSERT_EXCEPTION_CONTAINS(
+                    rootGroup2->Accept(TString(), TString(), *encoder),
+                    yexception,
+                    "label value is not valid UTF-8 string, name: 'validUtf8Name', value: 'invalid\\x80Utf8'"
+                );
+            }
+        }
     }
 
 }