opentelemetry traces otlp exporter

[TertiumOrganum1]

Distributed traces are must-have part of modern observability, especially for microservices. Currently there is no out-of-the-box method to use them in userver. The only way to get trace logs is to define logger with "opentracing" name and receive ltsv format. It would be nice to have a grpc or http trace exporter, just like in opentelemetry-cpp library. I tried to use them with userver but got strange seg faults, thus asking for similar functionality in userver.