[Monitoring] Fix bad metric value for UNTRIAGED_TESTCASE_COUNT (#4502)

Fixes the following error:

```
AttributeError
'ProtoType' object has no attribute 'DESCRIPTOR'
Failed to flush metrics: 'ProtoType' object has no attribute 'DESCRIPTOR' Traceback (most recent call last): File "/mnt/scratch0/clusterfuzz/src/clusterfuzz/_internal/metrics/monitor.py", line 118, in _flush_metrics metric.monitoring_v3_time_series(series, labels, start_time, end_time, File "/mnt/scratch0/clusterfuzz/src/clusterfuzz/_internal/metrics/monitor.py", line 350, in monitoring_v3_time_series self._set_value(point.value, value) File "/mnt/scratch0/clusterfuzz/src/clusterfuzz/_internal/metrics/monitor.py", line 404, in _set_value point.int64_value = value ^^^^^^^^^^^^^^^^^ File "/mnt/scratch0/clusterfuzz/src/third_party/proto/message.py", line 935, in __setattr__ pb_value = marshal.to_proto(pb_type, value) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/mnt/scratch0/clusterfuzz/src/third_party/proto/marshal/marshal.py", line 229, in to_proto proto_type.DESCRIPTOR.has_options ^^^^^^^^^^^^^^^^^^^^^ AttributeError: 'ProtoType' object has no attribute 'DESCRIPTOR'
```

src/clusterfuzz/_internal/cron/triage.py

@@ -322,7 +322,7 @@ def _increment_untriaged_testcase_count(job, status):
 def _emit_untriaged_testcase_count_metric():
   for (job, status) in untriaged_testcases:
     monitoring_metrics.UNTRIAGED_TESTCASE_COUNT.set(
-        untriaged_testcases, labels={
+        untriaged_testcases[(job, status)], labels={
             'job': job,
             'status': status,
         })