Add dynamically configurable sampling rules

[marcotc]

What does this PR do?

tracing_sampling_rules is now provided in the dynamic remote configuration payload, and configures the local trace sampling rules.

Also, each sampling rule has a new field, called provenance, which allows us to track through the decision-maker tag (_dd.p.dm) where is sampling decision came from. This even applies for locally configured rules, which get the default value of local provenance.

How to test the change?
There are units tests for all changes, plus system tests for integration testing: DataDog/system-tests#2361

Unsure? Have a question? Request a review!

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.51163% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 98.24%. Comparing base (36e37fc) to head (39a6034).

Files	Patch %	Lines
lib/datadog/tracing/configuration/dynamic.rb	82.35%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3598      +/-   ##
==========================================
- Coverage   98.24%   98.24%   -0.01%     
==========================================
  Files        1255     1255              
  Lines       74569    74641      +72     
  Branches     3540     3548       +8     
==========================================
+ Hits        73263    73332      +69     
- Misses       1306     1309       +3     

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[TonyCTHsu]

Since I have already compile the changelog for 2.0 in 2.0.0.beta1. Could you add this entry under the [Unreleased] section at Changelog.md.

[marcotc]

@TonyCTHsu would you like me to change https://github.com/DataDog/dd-trace-rb/blob/master/CHANGELOG.md directly in this PR?

[TonyCTHsu]

@TonyCTHsu would you like me to change https://github.com/DataDog/dd-trace-rb/blob/master/CHANGELOG.md directly in this PR?

That is also feasible 🎉