Set default endpoint for OTLP exporters

[trask]

I also went ahead and sync'd up the configuration settings between OTLP span and metrics exporters.

A common base class might be nice for those two builders in the future.

Closes #1463

[codecov]

Codecov Report

Merging #1464 into master will decrease coverage by 0.48%.
The diff coverage is 15.15%.

@@             Coverage Diff              @@
##             master    #1464      +/-   ##
============================================
- Coverage     87.35%   86.86%   -0.49%     
  Complexity     1328     1328              
============================================
  Files           162      162              
  Lines          5062     5094      +32     
  Branches        460      469       +9     
============================================
+ Hits           4422     4425       +3     
- Misses          472      496      +24     
- Partials        168      173       +5     

Impacted Files	Coverage Δ	Complexity Δ
...lemetry/exporters/otlp/OtlpGrpcMetricExporter.java	52.23% <12.90%> (-33.88%)	6.00 <0.00> (ø)
...telemetry/exporters/otlp/OtlpGrpcSpanExporter.java	70.14% <50.00%> (+0.45%)	5.00 <0.00> (ø)
...telemetry/sdk/trace/export/BatchSpanProcessor.java	94.59% <0.00%> (-1.36%)	8.00% <0.00%> (ø%)

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