Rack dependency should be explicit or optional

[stevehodgkiss]

I'd like to use this gem in a library that doesn't depend on Rack, but I get the following error when requiring dry/monitor:

LoadError: cannot load such file -- rack/utils
  /Users/steve/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/dry-monitor-0.1.2/lib/dry/monitor/rack/middleware.rb:1:in `require'
  /Users/steve/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/dry-monitor-0.1.2/lib/dry/monitor/rack/middleware.rb:1:in `<top (required)>'
  /Users/steve/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/dry-monitor-0.1.2/lib/dry/monitor.rb:3:in `require'
  /Users/steve/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/dry-monitor-0.1.2/lib/dry/monitor.rb:3:in `<top (required)>'

Should the dependency on Rack be optional? If it is then that needs taking into account when attempting to load the rack middleware. If not then rack should be added to the gemspec. I'm guessing the intention is for it to be an optional dependency. Maybe a separate gem would be better for that?

[GustavoCaso]

I think having rack as an optional dependency would be a right approach. WDYT? @solnic

[solnic]

Don't require entire dry/monitor if you don't want to use all of its features (which include rack middleware).