Implement ISupportExternalScope in SerilogLoggerProvider

[david-obee]

Addresses the issue #245.

This adds support in the SerilogLoggerProvider for the ISupportExternalScope interface. The Microsoft.Extensions.Logging library uses this interface when implemented to provide external scopes to logs. In practice, these scopes can include information about current Activity, such as the span ID, trace ID, tags, baggage, etc, which is configured by the ActivityTrackingOptions in the M.E.L library.

Support for tags and baggage via ActivityTrackingOptions was added in dotnet/runtime#48722.

I've left in the sample app SampleActivityLogging which I used to confirm this was possible, which also demonstrates that this change works correctly with the rest of M.E.L and indeed logs out tags and baggage.

If you were to run the SampleActivityLogging program before this change, you'd see the log line

{"Timestamp":"2024-03-13T15:46:52.6901279+00:00","Level":"Information","MessageTemplate":"Hello world!","TraceId":"d80a2e8ab54cfa140fff9cd2ec7458fd","SpanId":"f3f36fa6182fdb3b","Properties":{"SourceContext":"Program","Scope":["{ User = Hugh Mann, Time = 13/03/2024 15:46:52 +00:00 }"]}}

After this change, this is now:

{"Timestamp":"2024-03-13T15:29:20.8566575+00:00","Level":"Information","MessageTemplate":"Hello world!","TraceId":"4190732ed45816ada2d1153755f64415","SpanId":"2af23a9829387a6d","Properties":{"SourceContext":"Program","SpanId":"2af23a9829387a6d","TraceId":"4190732ed45816ada2d1153755f64415","ParentId":"0000000000000000","TraceState":null,"TraceFlags":"Recorded","tag.domain.id":1234,"baggage.environment":"uat","Scope":["{ User = Hugh Mann, Time = 13/03/2024 15:29:20 +00:00 }"]}}

where the I've switched on all the available ActivityTrackingOptions.

Outstanding questions

• There is already existing logic in Serilog to store the SpanId and TraceId on the LogEvent, and logic in this library in SerilogLogger to supply these values. I imagine we'd want to leave that alone for both backwards compatibility, and for users not using M.E.L? Instead, I think users should just not set those settings in ActivityTrackingOptions to avoid duplication.
• I've left in my extra sample project for the time being, so anyone can play around with this. I can remove this though should this PR get merged.

[nblumhardt]

Thanks, David.

This looks good to me. I'm a bit wary of how many allocations we seem to end up doing in that enrichment method - but that issue's separate from what your PR is addressing, so I'll look at it separately.

I think leaving the sample is fine, but perhaps it should be SampleWithExternalScope? Otherwise all LGTM 👍

[david-obee]

Thanks Nicholas, sounds good. I've updated the test project name.

[nblumhardt]

🎉