Feature for setting activity tags on creation

[rkargMsft]

Feature for adding tags at Activity creation time

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Summary of the changes

Adding a feature so that implementations can specify tags to add at Activity creation. This is to support using those tags when making sampling decisions as only tags provided in the CreateActivity/StartActivity are available at that time.

Description

Implementations of IHttpActivityCreationTagsFeature can determine what tags are desired to be used for sampling. It's important that this is an opt-in where the user specifies the tags to add since this work will get done for every Activity created for an incoming request and not just for the ones that eventually pass the sampling filter.

Example implementation:

public class BasicHttpActivityCreationTagsFeature : IHttpActivityCreationTagsFeature
{
    private readonly HttpContext _context;

    public BasicHttpActivityCreationTagsFeature(HttpContext context)
    {
        ArgumentNullException.ThrowIfNull(context);
        _context = context;
    }

    public IEnumerable<KeyValuePair<string, object?>>? ActivityCreationTags
    {
        get
        {
            if (_context?.Request is null)
            {
                return null;
            }

            var tags = new TagList();
            if (_context.Request.Host.HasValue)
            {
                tags.Add("server.address", _context.Request.Host.Host);
                if (_context.Request.Host.Port is not null)
                {
                    tags.Add("server.port", _context.Request.Host.Port);
                }
            }
            tags.Add("http.request.method", _context.Request.Method);
            tags.Add("url.scheme", _context.Request.Scheme);
            return tags;
        }
    }
}

Fixes #50488

[rkargMsft]

It may make sense to actually have the ActivityCreationTags property be IEnumerable<KeyValuePair<string, object?>>? which is what ActivityCreator takes as a parameter and then let the implementation decide what to return.

aspnetcore/src/Shared/Diagnostics/ActivityCreator.cs

Lines 13 to 22 in 6ab9fa6

	public static Activity? CreateFromRemote(
	ActivitySource activitySource,
	DistributedContextPropagator propagator,
	object distributedContextCarrier,
	DistributedContextPropagator.PropagatorGetterCallback propagatorGetter,
	string activityName,
	ActivityKind kind,
	IEnumerable<KeyValuePair<string, object?>>? tags,
	IEnumerable<ActivityLink>? links,
	bool diagnosticsOrLoggingEnabled)