Minimize sampling API

[marcotc]

2.0 Upgrade Guide notes

The following sampling classes have been removed from the public API:

Datadog::Tracing::Sampling::AllSampler
Datadog::Tracing::Sampling::Matcher
Datadog::Tracing::Sampling::SimpleMatcher
Datadog::Tracing::Sampling::ProcMatcher
Datadog::Tracing::Sampling::PrioritySampler
Datadog::Tracing::Sampling::RateByKeySampler
Datadog::Tracing::Sampling::RateByServiceSampler
Datadog::Tracing::Sampling::RateLimiter
Datadog::Tracing::Sampling::TokenBucket
Datadog::Tracing::Sampling::UnlimitedLimiter
Datadog::Tracing::Sampling::RateSampler
Datadog::Tracing::Sampling::Rule
Datadog::Tracing::Sampling::SimpleRule
Datadog::Tracing::Sampling::RuleSampler
Datadog::Tracing::Sampling::Sampler

Most custom sampling can be accomplished with a combination of Ingestion Controls and user-defined rules. This is the preferred option, as it is more maintainable and auditable than custom sampling.

If you still need a custom sampler, its API has been simplified in 2.0. See https://github.com/DataDog/dd-trace-rb/blob/2.0/docs/GettingStarted.md#custom-sampling for the new detailed requirements of a custom sampler.

Motivation:

We have made large improvements to sampling since 1.0.0 and plan to introduce new features and improvements in the near future.
The sampling changes introduced since 1.0.0 were hard to implement because almost all of the sampling classes are part of the public API. Moreover, large technical debt was introduced in order to keep the API stable while implementing such improvements. Thus, recent changes to the sampling logic have been very hard to implement correctly.

Since 1.0.0, we have introduced new ways to configure rule sampling, as well as post-ingestion sampling, which give users more maintainable options than using our custom sampling classes.

How to test the change?
There is no change in behaviour for ddtrace.
Changes to custom samplers are documented in our public docs.

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[codecov-commenter]

Codecov Report

Attention: 33 lines in your changes are missing coverage. Please review.

Comparison is base (5cb48c5) 98.13% compared to head (1aa01b2) 98.09%.
Report is 26 commits behind head on 2.0.

Files	Patch %	Lines
...datadog/tracing/contrib/trilogy/instrumentation.rb	41.30%	27 Missing ⚠️
lib/datadog/tracing/contrib/trilogy/patcher.rb	80.00%	3 Missing ⚠️
.../tracing/contrib/trilogy/configuration/settings.rb	96.77%	1 Missing ⚠️
lib/datadog/tracing/contrib/trilogy/integration.rb	95.23%	1 Missing ⚠️
.../datadog/tracing/contrib/rails/support/database.rb	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              2.0    #3423      +/-   ##
==========================================
- Coverage   98.13%   98.09%   -0.04%     
==========================================
  Files        1249     1254       +5     
  Lines       72086    72186     +100     
  Branches     3372     3379       +7     
==========================================
+ Hits        70741    70813      +72     
- Misses       1345     1373      +28     

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[brett0000FF]

Looks good from Docs side.