Sampling-Propagation:Set sampling decision mechanism tag

[marcotc]

Follow up from #2266

This PR adds a trace-level tag to propagate the sampling mechanism used to make a sampling decision: _dd.p.dm.

The currently defined mechanisms are (many unused):

        module Mechanism
          # Used before the tracer receives any rates from agent and there are no rules configured.
          DEFAULT = 0
          # The sampling rate received in the agent's http response.
          AGENT_RATE = 1
          # Auto. Reserved for future use.
          REMOTE_RATE_AUTO = 2
          # Sampling rule or sampling rate based on tracer config.
          TRACE_SAMPLING_RULE = 3
          # User directly sets sampling priority via {Tracing.reject!} or {Tracing.keep!},
          # or by a custom sampler implementation.
          MANUAL = 4
          # Formerly AppSec.
          ASM = 5
          # User-defined target. Reserved for future use.
          REMOTE_RATE_USER = 6
          # Reserved for future use.
          REMOTE_RATE_BY_DATADOG = 7
          # Single Span Sampled.
          SPAN_SAMPLING_RATE = 8
        end

The sampling mechanism can either come from an upstream service, if the sampling decision was performed upstream, or added by one of ddtraces sampling classes.

This PR also adds many feature test around different sampling mechanism (e.g. manual sampling, agent-based rates) in order to test for the newly added sampling mechanism tag. This unveiled a few bugs that have been fixed in separated PRs.

_dd.p.dm serialization format

One quirk of the implementation is due to the historical format of _dd.p.dm on other tracers: _dd.p.dm used to be comprised of two pieces of information, separated by a dash (-), e.g. info1-sampling_mechanism.

info1 is temporarily removed, by sampling_mechanism stays. This means that _dd.p.dm will look like this: -sampling_mechanism.

This ensures users using old versions of Datadog products that support the info1-sampling_mechanism format can still participate in the sampling propagation chain.

[codecov-commenter]

Codecov Report

Merging #2282 (627c116) into feat-Sampling-Propagation (8937ec9) will increase coverage by 0.00%. The diff coverage is 99.39%.

@@                     Coverage Diff                     @@
##           feat-Sampling-Propagation    #2282    +/-   ##
===========================================================
  Coverage                      97.59%   97.59%            
===========================================================
  Files                           1095     1095            
  Lines                          57535    57788   +253     
===========================================================
+ Hits                           56149    56396   +247     
- Misses                          1386     1392     +6     

Impacted Files	Coverage Δ
.../datadog/appsec/contrib/rack/request_middleware.rb	25.86% <0.00%> (-0.46%)	:arrow_down:
lib/datadog/tracing/sampling/span/ext.rb	100.00% <ø> (ø)
spec/datadog/tracing/sampling/rule_sampler_spec.rb	99.27% <88.88%> (+0.03%)	:arrow_up:
lib/datadog/appsec/event.rb	91.11% <100.00%> (+0.20%)	:arrow_up:
lib/datadog/core/configuration/components.rb	97.20% <100.00%> (ø)
lib/datadog/tracing/flush.rb	96.55% <100.00%> (ø)
lib/datadog/tracing/metadata/ext.rb	100.00% <100.00%> (ø)
lib/datadog/tracing/sampling/ext.rb	100.00% <100.00%> (ø)
lib/datadog/tracing/sampling/priority_sampler.rb	96.42% <100.00%> (+0.97%)	:arrow_up:
...ib/datadog/tracing/sampling/rate_by_key_sampler.rb	89.58% <100.00%> (-0.22%)	:arrow_down:
... and 25 more

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