fix:Convert custom setting value to string for startup log

[marcotc]

This PR ensures that integration settings that can store arbitrary values, e.g. "integration_rack_application", don't expand into a large payload like:

#<RailsApp::Application:0x00007fde60c43bc8
 @_all_autoload_paths=
  ["./rails/app/controllers",
   "./rails/app/helpers",
   "./rails/app/jobs",
   "./rails/app/models"],
 @_all_load_paths=
  ["./rails/app/controllers",
   "./rails/app/helpers",
   "./rails/app/jobs",
   "./rails/app/models"],
 @app=
  #<Datadog::Contrib::Rack::TraceMiddleware:0x00007fbfb4dda128
   @app=
    #<ActionDispatch::HostAuthorization:0x00007fbfb4dda240
     @app=
      #<Rack::Sendfile:0x00007fbfb4dda308
       @app=
        #<ActionDispatch::Executor:0x00007fbfb4dda380
         @app=
          #<ActiveSupport::Cache::Strategy::LocalCache::Middleware:0x00007fbfb4f3e000
           @app=
            #<Rack::Runtime:0x00007fbfb4dda3d0
             @app=
              #<Rack::MethodOverride:0x00007fbfb4dda3f8
               @app=
...

We currently call to_json on our startup log hash, thus calling to_hash on each object.
Because objects can have custom to_json handlers, it is not safe to perform that call.
For a Rails application, for example, you can get a SystemStackError due to recursive object resolution during to_json.

Instead they create a less descriptive, yet safer output using #to_s: "integration_rack_application":"#RailsApp::Application:0x00007fde60c43bc8"