Add public constants for all exporter default configuration values

[trask]

I found myself wanting these when reviewing: open-telemetry/opentelemetry-java-instrumentation#729.

And @anuraaga brought up here on my PR also: #1464 (comment)

[jkwatson]

@trask does #1486 work for this? If so, please close. Thanks!

[trask]

yes it does!