Add log message when stacktrace cannot be obtained for thread

This is to provide better diagnostics for 01051_system_stack_trace
failure [1].

  [1]: https://clickhouse-test-reports.s3.yandex.net/20881/866dfaec793f764dc9ba167d3ac9f6521b9b3381/functional_stateless_tests_(release,_wide_parts_enabled).html#fail1

src/Storages/System/StorageSystemStackTrace.cpp

@@ -16,6 +16,7 @@
 #include <Common/PipeFDs.h>
 #include <Common/CurrentThread.h>
 #include <common/getThreadId.h>
+#include <common/logger_useful.h>
 
 
 namespace DB
@@ -150,6 +151,7 @@ namespace
 
 StorageSystemStackTrace::StorageSystemStackTrace(const StorageID & table_id_)
     : IStorageSystemOneBlock<StorageSystemStackTrace>(table_id_)
+    , log(&Poco::Logger::get("StorageSystemStackTrace"))
 {
     notification_pipe.open();
 
@@ -229,6 +231,8 @@ void StorageSystemStackTrace::fillData(MutableColumns & res_columns, const Conte
         }
         else
         {
+            LOG_DEBUG(log, "Cannot obtain a stack trace for thread {}", tid);
+
             /// Cannot obtain a stack trace. But create a record in result nevertheless.
 
             res_columns[0]->insert(tid);

src/Storages/System/StorageSystemStackTrace.h

@@ -6,6 +6,10 @@
 #include <ext/shared_ptr_helper.h>
 #include <Storages/System/IStorageSystemOneBlock.h>
 
+namespace Poco
+{
+class Logger;
+}
 
 namespace DB
 {
@@ -30,6 +34,8 @@ protected:
     void fillData(MutableColumns & res_columns, const Context & context, const SelectQueryInfo & query_info) const override;
 
     mutable std::mutex mutex;
+
+    Poco::Logger * log;
 };
 
 }