How do I troubleshoot missing data in my Prometheus database?
I think your can do some kind of alerting on a metric rate with something like this:

ALERT DropInMetricsFromExporter
  IF rate(&lt;metric_name&gt;[1m]) == 0
  FOR 3m
  ANNOTATIONS {
    summary = "Rate of metrics is 0 {{ $labels.&lt;your_label&gt; }}",
    description = "Rate of metric dropped, actually: {{ $value }}%",
}


The main idea is to alert whenever the metric rate is at 0 for 3 minutes, with the proper metric name and a label somewhere telling from which exporter it comes it should give you the correct information.  

Choosing the right metric to monitor by exporter could be complex, without more insight is hard to give a better advice out of vacuum.
This blog post could be an inspiration also for a more generic detection.