Add Rails component configuration

[delner]

This PR depends on #809.

As a follow-up to #450 and #764, this pull request adds block style configuration for Rails components available, to make configuration more expressive and customizable.

Some new configuration examples include:

# Customize ActiveRecord with block
Datadog.configure do |c|
  c.use :rails do |rails|
    rails.service_name = 'my-app'

    rails.active_record do |active_record|
      active_record.service_name = "#{rails.service_name}-postgres"
    end
  end
end

# Customize different ActiveRecord databases with block
Datadog.configure do |c|
  c.use :rails do |rails|
    rails.service_name = 'my-app'

    rails.active_record describes: :primary_db do |active_record|
      active_record.service_name = "#{rails.service_name}-primary-db"
    end

    rails.active_record describes: :secondary_db do |active_record|
      active_record.service_name = "#{rails.service_name}-secondary-db"
    end
  end
end

# Deactivate ActiveRecord
Datadog.configure do |c|
  c.use :rails do |rails|
    rails.active_record false
  end
end

The Rails configuration now allows all its sub-integrations (rack, active_record, action_view, action_pack, active_support) and their constituent options to configured in this manner.

[delner]

This is considered a WIP; still need to add unit and integration tests for this one.

[marcotc]

Just to clarify, the describes: keyword argument of your active_record example,

rails.active_record describes: :primary_db do |active_record|

is not functional yet, is it?

[delner]

describes: is actually a valid, functional option. It's the multiplexer that allows users to define what configuration they want the settings to apply to. In the context of ActiveRecord, this is the name of the database described in databases.yml that they want these trace settings to apply to.

See #451

[marcotc]

👏 Looks good!
This should be an easy approval after tests are implemented.

[yijia-zhan]

@delner Just want to say thank you for this PR to follow up with #764! Any chance this can get in soon? :)

[delner]

@yijia-zhan we got this most of the way complete, but hit a snag with how defaults are evaluated under certain contexts; I will likely have to rework some of the design here to make it work.

I'm unfortunately quite busy at the moment trying to get some features in order, but I will revisit this as soon as I have a chance.

[mrmason]

This would be really handy, I've got a number of Rails applications that connect out to other databases using ActiveRecord, but all of the config I've tried always puts everything under the same service - I'm assuming from the Rails.application.config.after_initialize.

@delner are you able to advise on any current way to get separate dbs traced differently in a rails app ?

This is what I'm trying to get working...

  c.use(:rails, service_name: api')
  c.use(:sneakers, service_name: 'api-workers')

  c.use(:active_record, describes: { adapter: 'mysql2',
                                     database: "legacy" },
        service_name: 'api-legacy')
  
  c.use(:active_record, describes: { adapter: 'mysql2',
                                     database: "forum" },
        service_name: 'api-forum')

The rails main database is postgres - and all of the queries come through to DD on that service.

[ivoanjo]

Since this is very outdated, shall we go ahead and close it for now?