tracer autodetection for client interceptors

[funny-falcon]

Ability to automatically get tracer from request context if no tracer were passed to constructor.

[codecov-io]

Codecov Report

Merging #202 into master will decrease coverage by 0.55%.
The diff coverage is 18.75%.

@@            Coverage Diff             @@
##           master     #202      +/-   ##
==========================================
- Coverage   73.09%   72.54%   -0.56%     
==========================================
  Files          36       36              
  Lines        1349     1362      +13     
==========================================
+ Hits          986      988       +2     
- Misses        314      320       +6     
- Partials       49       54       +5

Impacted Files	Coverage Δ
tracing/opentracing/server_interceptors.go	75.55% <0%> (-3.52%)	⬇️
tracing/opentracing/client_interceptors.go	67.5% <20%> (-6.79%)	⬇️
tracing/opentracing/options.go	60% <25%> (-4.29%)	⬇️

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