Collecting statistics about dropped spans

[AlexanderWert]

As described in this spec, agents should collect statistics on dropped spans.

Spec issue

• [x] https://github.com/elastic/apm-server/issues/5851

Agent issues

• [x] https://github.com/elastic/apm-agent-java/issues/2084
• [x] https://github.com/elastic/apm-agent-dotnet/issues/1476
• [x] https://github.com/elastic/apm-agent-go/issues/1113
• [x] https://github.com/elastic/apm-agent-php/issues/536
• [ ] https://github.com/elastic/apm-agent-ruby/issues/1150
• [ ] https://github.com/elastic/apm-agent-nodejs/issues/2302
• [x] https://github.com/elastic/apm-agent-python/issues/1310

[gregkalapos]

Sorry, closed the wrong issue.