trace/exporters/ocagent: Retry logic check

[flands]

Per census-instrumentation/opencensus-service#134 we need to validate that exporters all behave consistently. Work is being tracked for each library:

• Add OC-Agent TraceService Java Exporter opencensus-java#1408
• agent-connection: asynchronously attempt to dial and retry census-ecosystem/opencensus-go-exporter-ocagent#18
• trace/exporters/ocagent: Implement retry logic and keep retrying in a daemon thread. opencensus-python#333

This issue is to track checking the Node implementation and determining what, if anything, needs to be done.

/cc @songy23