Unified tagging `rpc.system`

[TonyCTHsu]

What does this PR do?

Add tag rpc.system with value grpc for grpc integration

[codecov-commenter]

Codecov Report

Merging #2269 (1e5ad0f) into master (0e92706) will increase coverage by 0.00%. The diff coverage is 99.71%.

@@           Coverage Diff            @@
##           master    #2269    +/-   ##
========================================
  Coverage   97.56%   97.57%            
========================================
  Files        1087     1093     +6     
  Lines       56965    57292   +327     
========================================
+ Hits        55580    55901   +321     
- Misses       1385     1391     +6     

Impacted Files	Coverage Δ
.../datadog/tracing/contrib/rack/header_collection.rb	93.75% <93.75%> (ø)
lib/datadog/core/configuration/settings.rb	98.94% <100.00%> (+0.04%)	:arrow_up:
lib/datadog/core/header_collection.rb	100.00% <100.00%> (ø)
lib/datadog/tracing/client_ip.rb	100.00% <100.00%> (ø)
lib/datadog/tracing/configuration/ext.rb	100.00% <100.00%> (ø)
...tracing/contrib/grpc/datadog_interceptor/client.rb	97.77% <100.00%> (+0.05%)	:arrow_up:
...tracing/contrib/grpc/datadog_interceptor/server.rb	95.00% <100.00%> (+0.12%)	:arrow_up:
lib/datadog/tracing/contrib/rack/middlewares.rb	99.11% <100.00%> (+0.07%)	:arrow_up:
lib/datadog/tracing/metadata/ext.rb	100.00% <100.00%> (ø)
spec/datadog/core/header_collection_spec.rb	100.00% <100.00%> (ø)
... and 8 more

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