reader_concurrency_semaphore: add permit-stats

Which stores permit related stats. For now only total number of permits
is maintained which is useful to determine whether the semaphore was
used already or not.

reader_concurrency_semaphore.cc

@@ -168,6 +168,7 @@ struct reader_concurrency_semaphore::permit_list {
     using list_type = boost::intrusive::list<reader_permit::impl, boost::intrusive::constant_time_size<false>>;
 
     list_type permits;
+    permit_stats stats;
 };
 
 reader_permit::reader_permit(reader_concurrency_semaphore& semaphore, const schema* const schema, std::string_view op_name)
@@ -605,6 +606,7 @@ future<reader_permit::resource_units> reader_concurrency_semaphore::do_wait_admi
 void reader_concurrency_semaphore::on_permit_created(reader_permit::impl& permit) {
     _permit_gate.enter();
     _permit_list->permits.push_back(permit);
+    ++_permit_list->stats.total_permits;
 }
 
 void reader_concurrency_semaphore::on_permit_destroyed(reader_permit::impl& permit) noexcept {
@@ -612,6 +614,10 @@ void reader_concurrency_semaphore::on_permit_destroyed(reader_permit::impl& perm
     _permit_gate.leave();
 }
 
+reader_concurrency_semaphore::permit_stats reader_concurrency_semaphore::get_permit_stats() const {
+    return _permit_list->stats;
+}
+
 reader_permit reader_concurrency_semaphore::make_permit(const schema* const schema, const char* const op_name) {
     return reader_permit(*this, schema, std::string_view(op_name));
 }

reader_concurrency_semaphore.hh

@@ -79,6 +79,10 @@ public:
         // Total number of reads rejected because the admission queue reached its max capacity
         uint64_t total_reads_shed_due_to_overload = 0;
     };
+    struct permit_stats {
+        // Total number of permits created so far.
+        uint64_t total_permits = 0;
+    };
 
     struct permit_list;
 
@@ -282,6 +286,9 @@ public:
         return _stats;
     }
 
+    /// Return stats about the currently existing permits.
+    permit_stats get_permit_stats() const;
+
     /// Make a permit
     ///
     /// The permit is associated with a schema, which is the schema of the table