[Core] DiagnosticSource.Write needs type to work with trimming

[m-redding]

Context

In order to support The Open Telemetry Support for Native AOT as described here, we need to make the DiagnosticScope class compatible with trimming. This is a blocker for doing so.

The internal constructor for DiagnosticScope takes in the parameter object? diagnosticSourceArgs, see L30:

internal DiagnosticScope(string scopeName, DiagnosticListener source, object? diagnosticSourceArgs, ActivitySource? activitySource, System.Diagnostics.ActivityKind kind, bool suppressNestedClientActivities)

And passes it to the private ActivityAdapter class's constructor which sets it to a field object? _diagnosticSourceArgs, see L222:

public ActivityAdapter(ActivitySource? activitySource, DiagnosticSource diagnosticSource, string activityName, System.Diagnostics.ActivityKind kind, object? diagnosticSourceArgs)
{
...
          _diagnosticSourceArgs = diagnosticSourceArgs;

Issue

The issue is in L426 and L529:

_diagnosticSource.Write(_activityName + ".Start", _diagnosticSourceArgs ?? _currentActivity);

_diagnosticSource.Write(_activityName + ".Stop", _diagnosticSourceArgs);

Write() is not trim-friendly because it allows anything to be passed in as the payload. The way to get around this is to use a generic version of write() and preserve the type used for the payload (which is actually introduced in .NET 8+ but we can easily implement the same thing as a method). In addition, the type used in the type parameter needs to be annotated correctly so that the trimmer can preserve the right values.

Examples

There are examples of this here:

• Annotated type to preserve properties
• Suppressing the warning

Additional Context

Since _diagnosticSourceArgs is of type object, we need to figure out some way to preserve the type being passed in.

The constructor for DiagnosticScope is only used in 2 places outside of tests, one in the DiagnosticScopeFactory, and one in RequestActivityPolicy.

DiagnosticScopeFactory passes null in for diagnosticSourceArgs:

 var scope = new DiagnosticScope(
                                scopeName: name,
                                source: _source,
                                diagnosticSourceArgs: null,
                                activitySource: GetActivitySource(_source.Name, name),
                                kind: kind,
                                suppressNestedClientActivities: _suppressNestedClientActivities);

RequestActivityPolicy passes in HttpMessage

using var scope = new DiagnosticScope("Azure.Core.Http.Request", s_diagnosticSource, message, s_activitySource, System.Diagnostics.ActivityKind.Client, false);

Options

1. Change diagnosticSourceArgs from object? to HttpMessage?
2. Annotate HttpMessage and dismiss the warning. Document/Log heavily that if something besides HttpMessage is being passed in then things can get trimmed away (this is not ideal)
3. Create some kind of args class like the following:

namespace Azure.Core.Pipeline;

internal class DiagnosticSourceArgs
{
    public object Value { get; private set; }
    public void AddValue<T>(T value)
    {
        Value = value;
    }
    public void WriteToSource(DiagnosticSource source, string name)
    {
        source.Write(name, value);
    }

}

Non-options

1. Making DiagnosticScope generic