feat: delay sampling decision to allow sampling on tags and resource

[ZStriker19]

What does this PR do?
This PR enables lazy sampling with the feature being the ability to sample on tags and resource values for root spans. It does this by:

1. Removing sampling at span start
2. Change the metadata values returned by TraceOperation so that values from the root span will be returned if available, allowing sampling on all tags that will end up attached to the root span.
3. Running sampling in TraceOperation.to_digest which runs before context propagation
4. Running sampling in Tracer.flush which runs before spans are written to the agent.

This PR also fixes wildcard rule sampling when multiple * characters are repeated (e.g. ***), which are now considered equivalent to *, meaning "match all".

Motivation:
The ability to sample on tags and resource will be helpful to users trying to reduce costs in a precise way as well as keep those spans that are truly useful to them.

How to test the change?
Added tests for new behaviors and passes system-tests trace_sampling.py

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-10-07 17:50:36

Comparing candidate commit bc81964 in PR branch zachg/lazy_sampling with baseline commit 94cfe27 in branch master.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 23 metrics, 2 unstable metrics.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.43750% with 2 lines in your changes missing coverage. Please review.

Project coverage is 97.85%. Comparing base (56038a3) to head (bc81964).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/datadog/tracing/tracer.rb	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3956      +/-   ##
==========================================
- Coverage   97.86%   97.85%   -0.01%     
==========================================
  Files        1313     1313              
  Lines       78369    78477     +108     
  Branches     3888     3891       +3     
==========================================
+ Hits        76694    76797     +103     
- Misses       1675     1680       +5     

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[marcotc]

Awesome work, @ZStriker19!