Add caller information to main API for improved telemetry #103
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The message bus will already produce activity start/stop for key API invocations (Execute/Notify). This telemetry currently includes the recommended span code attributes, but it will log the Merq locations for those.
This means that monitoring the activities for a message bus Execute/Notify invocation will reveal source attributes that belong to Merq, rather than the actual place where those invocations happened. It would be much more useful to have the telemetry attributes be associated with the caller than the bus itself in these cases. By propagating these automatically (for example, a bus.Execute where the command handler performs bus.Notify) we can precisely know where a given event handler delivery action originated in the actual upstream call to the bus itself, revealing a key codepath that can be invaluable for diagnostics.
From the point of view of the activity and telemetry, at least with regards to these source code attributes, the bus becomes "transparent" in a way. This is more useful than monitoring method and line # for the bus implementation itself.
Closes #102