Add kafka reporter.

[jmjoy]

Add optional kafka reporter support.

• Move CollectItemProduce and CollectItemConsume from report::grpc module to report.
• Implement kafka reporter.
• Update documents.
• Add integration tests.

[codecov-commenter]

Codecov Report

Merging #61 (dd2ba29) into master (302526e) will decrease coverage by 2.14%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
- Coverage   16.88%   14.74%   -2.14%     
==========================================
  Files          18       20       +2     
  Lines         622      712      +90     
==========================================
  Hits          105      105              
- Misses        517      607      +90     

Impacted Files	Coverage Δ
e2e/src/e2e_kafka.rs	0.00% <0.00%> (ø)
src/proto/v3/mod.rs	0.00% <ø> (ø)
src/reporter/grpc.rs	0.00% <ø> (ø)
src/reporter/kafka.rs	0.00% <0.00%> (ø)
src/reporter/mod.rs	0.00% <0.00%> (ø)

... and 2 files with indirect coverage changes

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