Measure number of dropped spans in `BatchSpanProcessor`

Is your feature request related to a problem?
We would like to measure the number of dropped spans with the BatchSpanProcessor, simple True / False flag is not sufficient, as some (low) numbers of drops might not indicate anything bad.

Describe the solution you'd like
Having a simple way to add a meter without having to override or wrap on_exit to see how many spans were dropped because of a full queue. Ideally, a call to a callback when spans are dropped due to a full queue.

Describe alternatives you've considered
Re-implementing the on_exit of BatchSpanProcessor with added on_drop callback with a meter or adding a meter directly there. Eventually wrapping the existing method by checking the _spans_dropped flag and changing it back to false after incrementing a meter. Both methods might interfere with future changes, the latter would increase the log rate.