Unified tagging span.kind as server and client

[zarirhamza]

What does this PR do?

Unified tagging span.kind as server and client, inspired by Opentelemetry

[marcotc]

👋 @TonyCTHsu, you working on similar initiatives, can you please take a look at this PR?

[codecov-commenter]

Codecov Report

Merging #2365 (74d982b) into master (af16512) will decrease coverage by 0.02%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #2365      +/-   ##
==========================================
- Coverage   98.34%   98.32%   -0.03%     
==========================================
  Files        1102     1102              
  Lines       58929    59027      +98     
==========================================
+ Hits        57954    58037      +83     
- Misses        975      990      +15     

Impacted Files	Coverage Δ
lib/datadog/core/configuration/settings.rb	97.51% <ø> (ø)
...filing/collectors/cpu_and_wall_time_worker_spec.rb	91.19% <71.42%> (-8.81%)	⬇️
...g/profiling/collectors/cpu_and_wall_time_worker.rb	97.14% <100.00%> (ø)
...tracing/contrib/grpc/datadog_interceptor/server.rb	95.55% <100.00%> (+0.10%)	⬆️
lib/datadog/tracing/contrib/mongodb/subscribers.rb	96.87% <100.00%> (+0.04%)	⬆️
lib/datadog/tracing/contrib/rack/middlewares.rb	99.23% <100.00%> (+0.01%)	⬆️
spec/datadog/profiling/native_extension_spec.rb	97.87% <100.00%> (+0.37%)	⬆️
...ng/contrib/grpc/datadog_interceptor/server_spec.rb	100.00% <100.00%> (ø)
...pec/datadog/tracing/contrib/mongodb/client_spec.rb	99.65% <100.00%> (+<0.01%)	⬆️
...adog/tracing/contrib/rack/integration_test_spec.rb	100.00% <100.00%> (ø)
... and 3 more

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