Multi-tenanted logging

Currently, `Corvus.Monitoring`'s design is fairly closely aligned with things like Application Insights, in which there is a single sink for diagnostic information across a range of services.

In a multi-tenanted world, where a service might support multiple customers, we currently have no means by which instrumentation could indicate that some particular piece of information could be made available to a particular customer. Currently the presumption is that diagnostic information is collected for the benefit of the service provider, and not for the service provider's customers.

https://github.com/marain-dotnet/Marain.Workflow/ includes a built-in action for generating a log entry. In a multi-tenanted world, that only makes sense if that log entry is available to the service consumer. But that would mean we'd need the diagnostic instrumentation APIs to have some way of saying "This information should be visible to this tenant", and that's not currently a concept we support.

Start with the Corvus.Monitoring instrumentation APIs and compare them with the built-in log-entry action in Marain.Workflow. Define the tenant-visibility model and affected API surface before identifying tests; the issue names no files or existing tests, so done requires an agreed design rather than an obvious isolated edit.

Written by the indexing model from the issue text.