flake: TestInflightDispatchesMetric

```
    metrics_test.go:342: 
        	Error Trace:	/home/runner/work/coder/coder/coderd/notifications/metrics_test.go:342
        	Error:      	Condition never satisfied
        	Test:       	TestInflightDispatchesMetric
```

seen here: https://github.com/coder/coder/actions/runs/11363823264/job/31608588543