Drop Spans Shorter than threshold

**Is your feature request related to a problem? Please describe.**
We are running a custom integration for an ERP Application with the Python APM Agent.

The problem is that some of the transactions are really long and are composed of thousands of spans, and we are facing problems to view the slow spans, the ones that really matters for us, without configuring ELASTIC_APM_TRANSACTION_MAX_SPANS with a larger number.

**Describe the solution you'd like**

Instead of that we want an option to configure the min duration of an span for be captured, something like the ELASTIC_APM_SPAN_FRAMES_MIN_DURATION option but for SPANS itself and if the SPAN is not longer than the duration configured it not be captured and no count as SPAN for the ELASTIC_APM_TRANSACTION_MAX_SPANS option configured number, sending only those longer spans.