This adds an option to the collector exporters `concurrencyLimit`.  If this is set and
the number of export operations is equal to the limit, additional export operations
will fail immediately.

This should be set in combination with the batch span processor be set such that the
concurrency limit would not be reached under "normal" circumstances - only if there
is an issue would spans start to be dropped.

This helps us cap the amount of memory & sockets used by the exporter if it is not
able to keep up with the data it is being provided.

This could happen if the local network (e.g. in a browser) or the remote collector
are too slow to handle all the activity.

 If we do not have this cap, and the exporter cannot keep up, resources such as
 memory and network sockets can be consumed without limit, causing crashes and
 other undesirable outcomes far worse than losing some telemetry data.

 This also updates the examples to use `BatchSpanProcessor` as I couldn't really
 think of any reason why you would want to use SimpleSpanProcessor in combination
 with the collector exporter.