Tracing: added OpenTelemetry integration test

The test verifies that span tree structure and status code are valid.
Speculative executions are run parallel to the main thread, so some
of them can finish only after query result has been returned.
Thus, in order to collect span data from entire request, we decided
to wait until all speculative executions end. The main thread uses
conditional variable `allEnded` to wait for them and lock is used
for concurrent mutation of activeSpans.