Sampling-Propagation:Exclude problematic existing tag

[marcotc]

The distributed tracing tag _dd.p.upstream_services was at one point populated by other language traces, but can contain privileged information that can leak to third party services when injected in a distributed context.

Although the Ruby tracer has never populated this tag, we must make sure we don't propagate this tag downstream if we ever receive it in a distributed tracing request context.

[codecov-commenter]

Codecov Report

Merging #2299 (cf147d8) into sampling-dm-propagation (a0670e4) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@                   Coverage Diff                    @@
##           sampling-dm-propagation    #2299   +/-   ##
========================================================
  Coverage                    98.34%   98.34%           
========================================================
  Files                         1102     1102           
  Lines                        58856    58865    +9     
========================================================
+ Hits                         57879    57890   +11     
+ Misses                         977      975    -2     

Impacted Files	Coverage Δ
lib/datadog/tracing/distributed/headers/datadog.rb	100.00% <100.00%> (ø)
...atadog/tracing/distributed/headers/datadog_spec.rb	100.00% <100.00%> (ø)
lib/datadog/core/diagnostics/environment_logger.rb	100.00% <0.00%> (+1.57%)	:arrow_up:

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