[META 497] Collecting statistics about dropped spans

As described in [this spec](https://github.com/elastic/apm/blob/master/specs/agents/handling-huge-traces/tracing-spans-dropped-stats.md), agents should collect statistics on dropped spans.

Meta issue: [![issue details](https://img.shields.io/endpoint?url=https%3A%2F%2Fgiss.app.elstc.co%2Fapi%2Fstatus%2Felastic%2Fapm%2F497)](https://github.com/elastic/apm/issues/497)