Instrumenting many insignificant layers adds overhead. These insignificant layers are unlikely to be areas a developer will optimize. This adds a persistant Histogram that logs a count of total autoinstrumented layers and the percent significant per-file name.

```
[09/19/19 10:55:36 -0600 Dereks-MacBook-Pro.local (66345)] DEBUG : AutoInstrument Layer Histograms: {"/Users/dlite/projects/scout/apm/app/controllers/application_controller.rb"=>
  {:total=>1774, :significant=>0.08},
 "/Users/dlite/projects/scout/apm/app/controllers/status_pages_controller.rb"=>
  {:total=>5, :significant=>0.2},
 "/Users/dlite/projects/scout/apm/app/controllers/checkin_controller.rb"=>
  {:total=>52, :significant=>0.23},
 "/Users/dlite/projects/scout/apm/app/controllers/deploys_controller.rb"=>
  {:total=>15, :significant=>0.2},
 "/Users/dlite/projects/scout/apm/app/controllers/apps_controller.rb"=>
  {:total=>57, :significant=>0.46},
 "/Users/dlite/projects/scout/apm/app/controllers/errors_controller.rb"=>
  {:total=>4, :significant=>1.0},
 "/Users/dlite/projects/scout/apm/app/controllers/insights_controller.rb"=>
  {:total=>4, :significant=>1.0},
 "/Users/dlite/projects/scout/apm/app/controllers/alerts_controller.rb"=>
  {:total=>8, :significant=>0.25},
 "/Users/dlite/projects/scout/apm/app/controllers/traces_controller.rb"=>
  {:total=>12, :significant=>0.42},
 "/Users/dlite/projects/scout/apm/app/controllers/endpoints_controller.rb"=>
  {:total=>22, :significant=>0.32}}
```

This adds a `auto_instruments_ignore` config option so files can be excluded from autoinstruments. By defaut this is an empty array. Example usage: `auto_instruments_ignore: ['application_controller']`.

Logging autoinstrument layers that are significant/insignicant + ignoring files