text: Title: Add count to spider_exceptions stats
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  ## Summary
  
  Today, we only increase the specific exception under `spider_exceptions`, e.g., [`spider_exceptions/AttributeError`](https://github.com/scrapy/scrapy/blob/master/scrapy/core/scraper.py#L250). We should also have a total count like we have for downloader, e.g., [`downloader/exception_count`](https://github.com/scrapy/scrapy/blob/master/scrapy/downloadermiddlewares/stats.py#L80).
  
  ## Motivation
  
  This information is useful when creating visualizations of stats.
  