Improve Activity API usability and OpenTelemetry integration (Part 2)

[tarekgh]

The whole design of the proposed APIs is in the issue dotnet/designs#134

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[terrajobst]

Video

Spec

• ActivityContext.IsRemote sounds like the instance is remoted, which isn't the case, but we can't think of a better name.
• Making Activity.Kind settable seems very unfortunate. Given we do this backward compat only, we should find another way of doing this without making one of the key properties mutable. One option is to disallow changing after it was observed (whatever that means). The preferred option would be to find a way that doesn't require mutation (constructor parameter, factory method etc). This part requries more design.
• Activity.Attributes seems unfortunate b/c it adds more concepts.
  • Add SetTag(string, object)
  • Add AddTag(string, object)
  • Add TagObjects
  • The existing Tags property should probably be marked as obsolete and return all convert all objects to string.
• ActivityAttributesCollection should be renamed to something ActivityTagsCollection
• ActivityListener.PregenerateNewRootId has complex policy so be it

namespace System.Diagnostics
{
    public readonly partial struct ActivityContext
    {
        // This constructor already exist but we are adding extra defaulted parameter for isRemote.
        public ActivityContext(ActivityTraceId traceId, ActivitySpanId spanId, ActivityTraceFlags traceFlags, string? traceState = null, isRemote = false);

        // This is the new property
        public bool IsRemote { get; }
    }
    public partial class Activity
    {
        public void AddTag(string key, object value);
        public void SetTag(string key, object value);
        public IEnumerable<KeyValuePair<string, object>> TagObjects { get; }
    }
    public readonly struct ActivityEvent
    {
        public ActivityEvent(string name);
        public ActivityEvent(string name, DateTimeOffset? timestamp = null, ActivityTagsCollection? tags = null);
    }
    public readonly partial struct ActivityLink  : IEquatable<ActivityLink>
    {
        public ActivityLink(ActivityContext context, ActivityTagsCollection? tags = null);
    }
    public class ActivityTagsCollection : IDictionary<string, object>
    {
        // Implements this[key] = value as deleting the key
        // Add(key, null) should throw if key already exists, otherwise no-op
        // Should meant that this[key] returns null for-non existing keys too
    }
    public sealed class ActivityListener
    {
        public bool AutoGenerateRootContextTraceId { get; set; }
    }
}