OTLP exporter: Let final retry error include last retryable error message

[jmacd]

This improves the error messages printed when an OTel-Go SDK experiences retryable errors that eventually time out. Significantly, this includes text from the final retryable error to help the user understand the why the timeout has been reached.

I briefly updates the OTel collector example main() to avoid blocking in dial, to demonstrate the problem. After this change:

2022/12/05 11:14:10 context deadline exceeded: rpc error: code = Unavailable desc = connection error: desc = "transport: Error while dialing dial tcp 127.0.0.1:4318: connect: connection refused"
2022/12/05 11:14:20 context deadline exceeded: rpc error: code = Unavailable desc = connection error: desc = "transport: Error while dialing dial tcp 127.0.0.1:4318: connect: connection refused"

Before this change:

2022/12/05 11:14:10 context deadline exceeded
2022/12/05 11:14:20 context deadline exceeded

Part of #3513.

[dashpole]

lgtm, but probably needs a changelog

[jmacd]

Thanks for the help @MadVikingGod.

[codecov]

Codecov Report

Merging #3514 (5d5859a) into main (4607516) will not change coverage.
The diff coverage is 100.0%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3514   +/-   ##
=====================================
  Coverage   77.9%   77.9%           
=====================================
  Files        163     163           
  Lines      11850   11850           
=====================================
  Hits        9235    9235           
  Misses      2417    2417           
  Partials     198     198           

Impacted Files	Coverage Δ
exporters/otlp/internal/retry/retry.go	98.2% <100.0%> (ø)
sdk/trace/batch_span_processor.go	80.2% <0.0%> (-1.8%)	⬇️
exporters/jaeger/jaeger.go	91.1% <0.0%> (+0.8%)	⬆️
exporters/otlp/otlptrace/otlptracehttp/client.go	77.5% <0.0%> (+1.0%)	⬆️