OTLP/HTTP support for Node

[mwear]

Is your feature request related to a problem? Please describe.

The collector exporter supports JSON over HTTP for browser, and gRPC for Node. Since we already have the work in place, it'd be easy enough to add additional transport options to the collector exporter for node.

Describe the solution you'd like

The Node collector exporter should default to gRPC as it does now, but provide configuration for sending JSON or Protobuf over HTTP.