Support sending arbitrary tracing events to Elastic APM Server

[ohadravid]

Is your feature request related to a problem? Please describe.
When using the Elasticsearch exporter, tracing events generated as part of a span are not sent to Elasticsearch.
For example, this code:

from opentelemetry import trace

tracer = trace.get_tracer(__name__)

with tracer.start_as_current_span("my_span") as span:
    span.add_event("my_event")

will only generate the my_span span, while my_event will be ignored.

This is line with this comment in the exporter, saying it's a TODO.

Describe the solution you'd like
I'd like those events to be sent to Elasticsearch, just like they are sent to other backends.

Describe alternatives you've considered
We currently work around this by doing something like def add_event(name): with tracer.start_as_current_span(name): pass in Python, but in Rust for example that's not as easy.

Thanks!

[cyrille-leclerc]

As Elastic APM is now exposing an OTLP intake that is intended to replace the Otel Collector Exporter for Elastic, this issue is now tracked on elastic/apm-server#4715