Add tests to the topology builder for tracing span handling

[bruceg]

In #16309 we discovered that registered metrics created by sources were not having the correct set of tags attached to them. These tags are normally added automatically through the use of a tracing span. The fixes for this problem (#16416 and #16439) did resolve the exhibited problem, but did not add any tests to guard against regression.

I propose adding a set of tests for the config/topology builder that will:

1. Create a set of mock source, transform, and sink components that emit both a registered event and a non-registered event. Use random naming to guard against static string issues.
2. Build a running topology out of them and run it long enough to have all the required events emitted at least once.
3. Observe all the metrics emitted by the topology and ensure they all have the necessary tags and correct values for the tags.