Add option to inherit service name from the parent span

[marcotc]

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What does this PR do?

This PR is the first step towards aligning the Ruby tracer with all other APM languages in inheriting the parent span's service name by default.

Making this change is a breaking change, so this is introduced as an option which default to "disabled".

This option should be made "enabled" by default in the next major version.

How to test the change?

Changing the setting c.tracing.inherit_parent_service to true allows you to see the new behaviour, with the span's service inheriting the value from the active parent span.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.10%. Comparing base (5a95611) to head (c2aea03).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3685   +/-   ##
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  Coverage   98.10%   98.10%           
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  Files        1225     1225           
  Lines       72846    72927   +81     
  Branches     3489     3490    +1     
=======================================
+ Hits        71463    71547   +84     
+ Misses       1383     1380    -3     

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