Add sampling by trace tags

[marcotc]

What does this PR do?
This PR expands the sampling rule engine to allow for matching on traces based on their tags and metrics.
In ddtrace, both tags (string value) and metrics (float value) are called tags, but the matcher has a few implementation details that makes it work with both.

How to test the change?

There are unit tests in this PR, as well as system tests: https://github.com/DataDog/system-tests/blob/373c58b012da02794385510cb083001f5a0b9013/tests/parametric/test_trace_sampling.py#L302

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.63014% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 98.25%. Comparing base (ce32588) to head (1cd50e0).
Report is 12 commits behind head on master.

Files	Patch %	Lines
spec/datadog/tracing/sampling/rule_spec.rb	91.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #3587    +/-   ##
========================================
  Coverage   98.24%   98.25%            
========================================
  Files        1254     1255     +1     
  Lines       74375    74545   +170     
  Branches     3529     3537     +8     
========================================
+ Hits        73073    73241   +168     
- Misses       1302     1304     +2     

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[TonyCTHsu]

LGTM