Rename TracerRegistry -> Tracer Provider

[thisthat]

This addresses #794

Specification: link

[codecov-io]

Codecov Report

Merging #879 into master will increase coverage by 0.06%.
The diff coverage is 95%.

@@             Coverage Diff              @@
##             master     #879      +/-   ##
============================================
+ Coverage     82.89%   82.96%   +0.06%     
- Complexity      834      835       +1     
============================================
  Files           108      108              
  Lines          2865     2865              
  Branches        253      253              
============================================
+ Hits           2375     2377       +2     
+ Misses          376      375       -1     
+ Partials        114      113       -1

Impacted Files	Coverage Δ	Complexity Δ
...lemetry/sdk/common/InstrumentationLibraryInfo.java	100% <ø> (ø)	3 <0> (ø)	⬇️
...o/opentelemetry/sdk/trace/export/SpanExporter.java	100% <ø> (ø)	0 <0> (ø)	⬇️
.../io/opentelemetry/sdk/trace/TracerSharedState.java	93.33% <ø> (ø)	10 <0> (ø)	⬇️
...ain/java/io/opentelemetry/sdk/trace/TracerSdk.java	100% <ø> (ø)	9 <0> (ø)	⬇️
.../opentelemetry/sdk/trace/spi/TraceProviderSdk.java	100% <100%> (ø)	2 <1> (ø)	⬇️
...in/java/io/opentelemetry/sdk/OpenTelemetrySdk.java	100% <100%> (ø)	3 <1> (ø)	⬇️
.../io/opentelemetry/sdk/trace/TracerSdkProvider.java	100% <100%> (ø)	10 <2> (?)
.../io/opentelemetry/trace/DefaultTracerProvider.java	100% <100%> (ø)	5 <2> (?)
.../src/main/java/io/opentelemetry/OpenTelemetry.java	97.43% <100%> (ø)	15 <1> (ø)	⬇️
...va/io/opentelemetry/opentracingshim/TraceShim.java	100% <100%> (ø)	3 <2> (ø)	⬇️
... and 3 more

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