Add new mechanism to enable creation of DiagnosticContext scopes

[gkinsman]

This PR is a WIP proposal to fix #47. There's still a few unanswered questions below as I'm not sure this is 100% the right direction :).

It introduces a new method to IDiagnosticContext that allows users to begin a new DiagnosticContextScope that collects properties via IDiagnosticContext:

DiagnosticContextScope Begin(string messageTemplate, params object[] properties);

Open Questions

There's still a number of areas that haven't been fully thought through on the interface method:

• [ ] Configurable log levels (like with RequestLoggingMiddleware)?
• [ ] Additional enrichers for the completion event?
• [ ] Specify a custom logger to use?
• [ ] It's possible IDiagnosticContext isn't the right place for Begin, and that maybe it belongs on some other thing that should be injected to keep IDC clean. Putting it on ILogger might work, but then you'd need to pass in an IDiagnosticContext.
• [ ] Is adding this to IDiagnosticContext even an option? It's public so others could implement it, but there's no real way to use it from what I can tell.

Sample Usage

...
var operationId = Guid.NewGuid();
using (_diagnosticContext.Begin("Worker executed operation {OperationId} at time {Time}", operationId, DateTimeOffset.Now)) {
    await _executor.DoWork(operationId);
}
...

public async Task DoWork(Guid operationId)
{
    ...
    _diagnosticContext.Set("OperationName", randomOperation);
    _diagnosticContext.Set("ThisOperationId", operationId);
    ...
}
...

Which produces this event:

[nblumhardt]

Thanks for sending this; will have a dig in properly as soon as I have a chance (the last few months have been a bit crazy, here :-))

[gkinsman]

Thanks for sending this; will have a dig in properly as soon as I have a chance (the last few months have been a bit crazy, here :-))

No worries Nick! Hope everything's well 🙂

[nblumhardt]

@gkinsman snowed :-D but digging out hehe 👍