Add peer.service tag to Span

[qudongfang]

Add peer.service tag to Span so that dependency diagram can show jdbc middleware components.

1. https://github.com/jaegertracing/spark-dependencies/issues/70
2. https://github.com/jaegertracing/spark-dependencies/issues/69#issuecomment-517688562

[coveralls]

Pull Request Test Coverage Report for Build 170

• 9 of 9 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 45.329%

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Totals
Change from base Build 169:	0.5%
Covered Lines:	393
Relevant Lines:	867

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💛 - Coveralls