Add Logging for Instance Scrape Timeouts

[jdlafaye]

PR #60 introduced a per-instance scrape timeout. Currently there is no capability to determine which connected instances are timing out when scraped. For debugging purposes it would be helpful to log an occurrence of an instance timing out its scrape.

However, connected instances are currently only identified by a key pair and logging a public key to denote the instance that is failing its scrape is not particularly helpful for debugging. Thus, to produce log messages that are helpful for debugging we'll need to store additional information about connected instances. At a bare minimum this should probably include source IP address and port but could be even better if metadata/tags (denoting the app name, instance number, environment, etc. as chosen by the client) were included.

Not entirely sure how this would be implemented off the top of my head but I would think it would be possible to gather some of this data when a client connects and store it in the scrapableApps data structure for later logging when a timeout occurs.