Set spider name in the stats logs.

[roykyle8]

When the multiple crawlers are run through CrawlerProcess, logstats are not clear as which spider these stats are for. Adding a spider name in the log.

[wRAR]

I think fixing #3163 is better than adding the spider name to the message itself (and it's not specific to logstats).

[wRAR]

Hmm or is this a different problem? Do you only have problems with the logstats message when you run several spiders?

[roykyle8]

@wRAR Ideally we should have a spider name in every log otherwise its very difficult to find which spider the log relates to however even if its implemented, I'm not sure how it will be implemented into stats logger as stat separate classe linked to a spider instance and is not a part of spider.

[kmike]

It seems it'd be nice to include the spider name to all log messages, optionally. Probably, it's a matter of allowing to include spider_name variable in the LOG_FORMAT option (https://docs.scrapy.org/en/latest/topics/settings.html#log-format), when the spider name is available.