Rename newBuilder to builder for consistency.

[anuraaga]

We currently are not consistent with whether we use newBuilder or builder for a builder factory. This consolidates on builder, similar to Guava and slightly recommended by AutoValue.

[codecov]

Codecov Report

Merging #1790 into master will decrease coverage by 0.01%.
The diff coverage is 72.22%.

@@             Coverage Diff              @@
##             master    #1790      +/-   ##
============================================
- Coverage     85.94%   85.92%   -0.02%     
+ Complexity     1379     1378       -1     
============================================
  Files           166      166              
  Lines          5307     5307              
  Branches        549      549              
============================================
- Hits           4561     4560       -1     
  Misses          551      551              
- Partials        195      196       +1     

Impacted Files	Coverage Δ	Complexity Δ
...metry/exporters/inmemory/InMemorySpanExporter.java	95.45% <ø> (ø)	7.00 <0.00> (ø)
...metry/exporters/jaeger/JaegerGrpcSpanExporter.java	76.54% <ø> (ø)	4.00 <0.00> (ø)
...lemetry/exporters/otlp/OtlpGrpcMetricExporter.java	53.93% <0.00%> (ø)	6.00 <0.00> (ø)
...telemetry/exporters/otlp/OtlpGrpcSpanExporter.java	71.42% <0.00%> (ø)	5.00 <0.00> (ø)
...etry/exporters/prometheus/PrometheusCollector.java	100.00% <ø> (ø)	4.00 <0.00> (ø)
...telemetry/exporters/zipkin/ZipkinSpanExporter.java	87.96% <ø> (ø)	30.00 <0.00> (ø)
...o/opentelemetry/extensions/trace/MessageEvent.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
.../main/java/io/opentelemetry/SendTraceToJaeger.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...telemetry/sdk/trace/export/BatchSpanProcessor.java	89.18% <ø> (ø)	8.00 <0.00> (ø)
...elemetry/sdk/trace/export/SimpleSpanProcessor.java	92.30% <ø> (ø)	12.00 <0.00> (ø)
... and 13 more

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[jkwatson]

Thanks! Consistency is important in the API to reduce confusion.