Add OpenTelemetry Collector exporter for web

[mayurkale22]

Add OpenTelemetry Collector (aka "agent") exporter for Web SDK. The OpenTelemetry Collector can receive traces and metrics from processes instrumented by OpenTelemetry and exports traces and metrics to one or more monitoring/tracing backends. One of the advantage of this approach is that you don't have to write all exporters in native language.

The best reference architecture would be Browser > OpenTelemetry Collector Exporter > OpenTelemetry Collector > Destination(s)

Reference code: https://github.com/census-instrumentation/opencensus-web/tree/master/packages/opencensus-web-exporter-ocagent

[mayurkale22]

/cc @draffensperger @obecny

[mayurkale22]

Please suggest name for the package. One suggestion: opentelemetry-exporter-web-collector

opentelemetry-exporter-* is matching with other exporters name pattern, web is to indicate that this is meant for browser and collector is the backend name similat to zipkin/jaeger.

[obecny]

opentelemetry-exporter-web-collector sounds most reasonable for me

[draffensperger]

I agree with opentelemetry-exporter-web-collector, though if it ends up that there can be a lot of shared code between Node and web here then having a single opentelemetry-exporter-collector might make sense - but let's start with two separate packages.

[obecny]

it appeared to be easier to create web and node at the same time. I named this as
opentelemetry-exporter-collector and class CollectorExporter