Sampling test

[GoncaloGarcia]

With sampling we can reduce the tracing overhead by limiting the
percentage of requests that are traced.

Since Jaeger's sampling functionality does not ignore the
instrumentation code we had to implement our own sampling.

This is done by generating a thread local random every time we call
addToTrace and if this value is below the sampling threshold a baggage
item ("sampled" : "True") is added to the span. From then on if the
active span's baggage does not contain that item, the instrumentation
code will be ignored and when necessary Noop versions of Span, Scope and
SpanContext are returned.

[codecov]

Codecov Report

Merging #16 into master will decrease coverage by 11.09%.
The diff coverage is 41.91%.

@@             Coverage Diff              @@
##             master      #16      +/-   ##
============================================
- Coverage     71.27%   60.17%   -11.1%     
- Complexity       90       92       +2     
============================================
  Files             7       16       +9     
  Lines           369      452      +83     
  Branches         23       25       +2     
============================================
+ Hits            263      272       +9     
- Misses           85      150      +65     
- Partials         21       30       +9

Impacted Files	Coverage Δ	Complexity Δ
...acing/engine/opentracing/noop/NoopSpanBuilder.java	0% <0%> (ø)	0 <0> (?)
...g/engine/opentracing/noop/NoopSpanContextImpl.java	0% <0%> (ø)	0 <0> (?)
...mons/tracing/engine/opentracing/noop/NoopSpan.java	0% <0%> (ø)	0 <0> (?)
...racing/engine/opentracing/noop/NoopTracerImpl.java	0% <0%> (ø)	0 <0> (?)
...g/engine/opentracing/noop/NoopSpanBuilderImpl.java	0% <0%> (ø)	0 <0> (?)
.../engine/opentracing/noop/NoopScopeManagerImpl.java	0% <0%> (ø)	0 <0> (?)
.../tracing/engine/opentracing/noop/NoopSpanImpl.java	0% <0%> (ø)	0 <0> (?)
...cing/engine/opentracing/noop/NoopScopeManager.java	0% <0%> (ø)	0 <0> (?)
...ing/engine/opentracing/noop/NoopTracerFactory.java	0% <0%> (ø)	0 <0> (?)
...racing/engine/configuration/BaseConfiguration.java	100% <100%> (ø)	4 <4> (?)
... and 13 more

---

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[GoncaloGarcia]

There might be a small issue with this pull request as it relies on the OpenTracing Noop implementation which isn't public (the API is, but the implementation classes aren't) so I had to download the code from their Github and include it in ours (making the implementation classes public in the process). Not sure how licensing works here but they use Apache 2.0 as far as I know.

[codecov-io]

Codecov Report

Merging #16 into master will decrease coverage by 11.09%.
The diff coverage is 41.91%.

@@             Coverage Diff              @@
##             master      #16      +/-   ##
============================================
- Coverage     71.27%   60.17%   -11.1%     
- Complexity       90       92       +2     
============================================
  Files             7       16       +9     
  Lines           369      452      +83     
  Branches         23       25       +2     
============================================
+ Hits            263      272       +9     
- Misses           85      150      +65     
- Partials         21       30       +9

Impacted Files	Coverage Δ	Complexity Δ
...acing/engine/opentracing/noop/NoopSpanBuilder.java	0% <0%> (ø)	0 <0> (?)
...g/engine/opentracing/noop/NoopSpanContextImpl.java	0% <0%> (ø)	0 <0> (?)
...mons/tracing/engine/opentracing/noop/NoopSpan.java	0% <0%> (ø)	0 <0> (?)
...racing/engine/opentracing/noop/NoopTracerImpl.java	0% <0%> (ø)	0 <0> (?)
...g/engine/opentracing/noop/NoopSpanBuilderImpl.java	0% <0%> (ø)	0 <0> (?)
.../engine/opentracing/noop/NoopScopeManagerImpl.java	0% <0%> (ø)	0 <0> (?)
.../tracing/engine/opentracing/noop/NoopSpanImpl.java	0% <0%> (ø)	0 <0> (?)
...cing/engine/opentracing/noop/NoopScopeManager.java	0% <0%> (ø)	0 <0> (?)
...ing/engine/opentracing/noop/NoopTracerFactory.java	0% <0%> (ø)	0 <0> (?)
...racing/engine/configuration/BaseConfiguration.java	100% <100%> (ø)	4 <4> (?)
... and 13 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8d79839...6a1c48e. Read the comment docs.