Targeted tests for trace building

Today we have very limited testing for trace building. `client.events` has tests for the trace client event. `client.null_instrument` only verifies whether a trace is built or not. Mostly we rely on implicit trace creation in other tests in our suite, which doesn't allow detection of bugs besides those that would cause crashes. We should add comprehensive testing for corner cases.

Start by reading the existing trace-client-event tests in `client.events` and the trace existence check in `client.null_instrument`. Identify the trace-building corner cases that are currently untested and add coverage beyond crash detection. Done means the relevant trace-building behavior is exercised by targeted tests and the test suite passes.

Written by the indexing model from the issue text.