Add tracing span around Allow()'s Redis call

[spenceral]

This change adds observability to the hot path in quota service
deployments that use Redis as the bucket cache. It surrounds the Redis
EvalSha call with an OpenTracing span so that distributed tracing
systems can be used to see how much of Allow()'s execution time is spent
in the Redis call.

[maniksurtani]

LGTM once you sort out the dependencies.

[coveralls]

Pull Request Test Coverage Report for Build 805

• 3 of 3 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 68.097%

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Totals
Change from base Build 790:	0.0%
Covered Lines:	476
Relevant Lines:	699

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 805

• 3 of 3 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 68.097%

---

Totals
Change from base Build 790:	0.0%
Covered Lines:	476
Relevant Lines:	699

---

💛 - Coveralls

[spenceral]

Rebased to bring in @szabado's Travis fixes and modules change. Thanks for those Felix!