Add flush interval for Jaeger exporter

[bg451]

During the SIG meeting today, @mayurkale22 mentioned a possible bug with the Jaeger exporter where Jaeger client only flushes when its span buffer reaches a size threshold, meaning that if you have a very low throughput system it might take a very long time for spans to get sent to Jaeger.

Looking at code, the exporter uses Jaeger's UDPSender, which is a fairly simple, low level piece of code that gets wrapped with more complex logic. In the jaeger client code base, there's another class that accepts a sender called a RemoteReporter which actually implements the interval flushing logic.

We can either add a flush interval to the Jaeger exporter or use RemoteReporter. I'm leaning towards adding a flush interval since it's a very small amount of code. Thoughts?

[mayurkale22]

+1 for flush interval option.

Another option could be, when we done with append logic just trigger the flush call https://github.com/open-telemetry/opentelemetry-js/blob/master/packages/opentelemetry-exporter-jaeger/src/jaeger.ts#L84-L92.

This should work nicely with both SpanProcessors (BatchSpanProcessor and SimpleSpanProcessor). In case of BatchSpanProcessor, append+flush will fire only when reached the max buffer size or timeout limit and in case of SimpleSpanProcessor, I expect to see immediately export (which won't work if we go with flush interval or may be we need to set small interval number).

Also, with this we can avoid multiple interval logic. This is the one in BatchSpanProcessor -> https://github.com/open-telemetry/opentelemetry-js/blob/master/packages/opentelemetry-basic-tracer/src/export/BatchSpanProcessor.ts#L47.