Add toString to OTLP exporters

[jack-berg]

#5680 and #5652 opened the door to an easy toString implementation on OTLP exporters. This continues down the path of making it easier to diagnose the configuration of OpenTelemetrySdk, since the otel java agent or any other application can log OpenTelemetrySdk#toString().

Another nice side affect of this is that it makes it easier to compare configured instances of the OTLP exporters (i.e. from file configuration) without a bunch of reflective introspection to examine internal fields.

Note, in order to avoid printing secrets, I print all headers as {header-name}=OBFUSCATED. Better to be overly restrictive about what we print than accidentally print some secret because we misidentify a header as safe to print.

[codecov]

Codecov Report

Patch coverage: 82.92% and project coverage change: -0.04% ⚠️

Comparison is base (d4a3b3b) 91.34% compared to head (3aaa0e0) 91.31%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5686      +/-   ##
============================================
- Coverage     91.34%   91.31%   -0.04%     
- Complexity     4997     5010      +13     
============================================
  Files           554      554              
  Lines         14793    14834      +41     
  Branches       1379     1383       +4     
============================================
+ Hits          13513    13546      +33     
- Misses          883      887       +4     
- Partials        397      401       +4     

Files Changed	Coverage Δ
...ry/exporter/internal/http/HttpExporterBuilder.java	89.24% <76.47%> (-2.86%)	⬇️
...ry/exporter/internal/grpc/GrpcExporterBuilder.java	95.37% <83.33%> (-3.52%)	⬇️
...rter/otlp/http/logs/OtlpHttpLogRecordExporter.java	83.33% <100.00%> (+1.51%)	⬆️
...rter/otlp/http/metrics/OtlpHttpMetricExporter.java	87.50% <100.00%> (+0.83%)	⬆️
...exporter/otlp/http/trace/OtlpHttpSpanExporter.java	83.33% <100.00%> (+1.51%)	⬆️
.../exporter/otlp/logs/OtlpGrpcLogRecordExporter.java	83.33% <100.00%> (+1.51%)	⬆️
.../exporter/otlp/metrics/OtlpGrpcMetricExporter.java	87.50% <100.00%> (+0.83%)	⬆️
...etry/exporter/otlp/trace/OtlpGrpcSpanExporter.java	83.33% <100.00%> (+1.51%)	⬆️

... and 2 files with indirect coverage changes

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