Adding 'peer.service' tag indicating remote service

[peterjurkovic]

Based on Open Tracing Semantic Conventions there should be also peer.service tag:

Remote service name (for some unspecified definition of "service"). E.g., "elasticsearch", "a_custom_microservice", "memcache". Meaning should correspond with values set in service.

In this case "kafka". Based on this tag tracing implementors can render UI. Any reason why was not added?

[coveralls]

Pull Request Test Coverage Report for Build 117

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 57.764%

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Totals
Change from base Build 116:	0.4%
Covered Lines:	186
Relevant Lines:	322

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💛 - Coveralls

[malafeev]

thanks @peterjurkovic