Send span only if duration geater than a threshold (ELASTIC_APM_SPAN_MIN_DURATION)

**Is your feature request related to a problem? Please describe.**
I use agent on a website with a lot of db queries. I don't necessary need all span about that, just slower

**Describe the solution you'd like**
Be able to have an option like ELASTIC_APM_SPAN_MIN_DURATION
Perhaps be able to specify a threshold by span type (eg : ELASTIC_APM_SPAN_MIN_DURATION="mysql:5ms,http:50ms...'

**Additional context**
I tested a solution by implemented that on endSpanEx method by setted isDropped property to true
I didn't update code about options..., but if you think it's a good idea, I can do on a PR