cmd/apmtoo(tracegen): Return stats of otlp

It is expected for the client to know the stats of the trace generator, for example, how many events were generated by the agents. For apm-go agent, we pass the `Tracer` so it is possible for the client to get stats without needing the tracegen package to return the info. However, otlp doesn't take the same approach so it is impossible to know how many docs were generated by the otlp. We need to expose the stats of otlp.

Start at the cmd/apmtoo tracegen entry point and inspect how the OTLP generator differs from the apm-go agent's Tracer-based approach. Trace the OTLP path through the tracegen package and determine how the client could receive generated-document counts; done means OTLP statistics are exposed to the client.

Written by the indexing model from the issue text.