Add extra service configuration for RC

[lloeki]

What does this PR do?

Add extra service configuration for RC

Motivation

Service names can be mixed and matched in many ways, sometimes in complex ways that cannot be automatically inferred. For this information to be properly relayed in the most complex cases manual configuration is necessary.

Additional Notes

It may be possible to automate population of this field in the future (depends on configuration system evolution).

How to test the change?

Here's a typical configuration example, where:

• main service name is different from sub-services provided by integrations
• remote service name is made to match main service name
• rack and rails service names are different, and subject to remote configuration for this process as well
• request queuing is handled, creates a synthetic span which belongs to another service in another process, and so is not provided as extra service for RC.

  Datadog.configure do |c|
    c.service = 'web'

    c.tracing.enabled = true
    c.tracing.instrument :rack, service_name: 'web-rack', request_queuing: true, web_service_name: 'nginx'
    c.tracing.instrument :rails, service_name: 'web-rails'

    c.remote.enabled = true
    c.remote.service = 'web'
    c.remote.extra_services = ['web-rack', 'web-rails']
  end

[delner]

What is extra_services? It wasn't clear from my immediate read. I'm also curious how this lines up against some of our other service name changes.

[codecov-commenter]

Codecov Report

Merging #3032 (af340e2) into master (2557a9a) will decrease coverage by 0.01%. Report is 109 commits behind head on master. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3032      +/-   ##
==========================================
- Coverage   98.16%   98.16%   -0.01%     
==========================================
  Files        1283     1283              
  Lines       73915    73952      +37     
  Branches     3425     3428       +3     
==========================================
+ Hits        72560    72594      +34     
- Misses       1355     1358       +3     

Files	Coverage Δ
lib/datadog/core/configuration/settings.rb	98.54% <100.00%> (+0.06%)	:arrow_up:
lib/datadog/core/remote/client.rb	91.37% <100.00%> (+0.30%)	:arrow_up:
spec/datadog/core/configuration/settings_spec.rb	100.00% <100.00%> (ø)
spec/datadog/core/remote/client_spec.rb	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[ivoanjo]

I've discussed this with Loic, and since we ended up not using it, I'm going to go ahead and close this PR for now :)