Ensure baggage gets copied with baggage prefix

This brings our agent in line with the wider agent specification on how
to implement baggage propagation

https://github.com/elastic/apm/blob/main/specs/agents/tracing-distributed-tracing.md#baggage-related-configuration

src/Elastic.Apm/Api/Context.cs

@@ -12,12 +12,12 @@ namespace Elastic.Apm.Api
 {
 	public class Context
 	{
-		private Lazy<Dictionary<string, string>> _custom = new Lazy<Dictionary<string, string>>();
+		private Lazy<Dictionary<string, string>> _custom = new();
 
 		[JsonConverter(typeof(CustomJsonConverter))]
 		public Dictionary<string, string> Custom => _custom.Value;
 
-		internal Lazy<LabelsDictionary> InternalLabels = new Lazy<LabelsDictionary>();
+		internal Lazy<LabelsDictionary> InternalLabels = new();
 
 		/// <summary>
 		/// <seealso cref="ShouldSerializeLabels" />

src/Elastic.Apm/Model/Error.cs

@@ -73,7 +73,9 @@ private void CheckAndCaptureBaggage()
 				if (!WildcardMatcher.IsAnyMatch(Configuration.BaggageToAttach, baggage.Key))
 					continue;
 
-				Context.InternalLabels.Value.Add(baggage.Key, baggage.Value);
+				var newKey = $"baggage.{baggage.Key}";
+				var labels = Context.InternalLabels.Value;
+				labels[newKey] = baggage.Value;
 			}
 		}
 

src/Elastic.Apm/Model/Span.cs

@@ -138,8 +138,10 @@ private void CheckAndCaptureBaggage()
 				if (!WildcardMatcher.IsAnyMatch(Configuration.BaggageToAttach, baggage.Key))
 					continue;
 
-				Otel ??= new OTel() { Attributes = new Dictionary<string, object>() };
-				Otel.Attributes.Add(baggage.Key, baggage.Value);
+				Otel ??= new OTel { Attributes = new Dictionary<string, object>() };
+
+				var newKey = $"baggage.{baggage.Key}";
+				Otel.Attributes[newKey] = baggage.Value;
 			}
 		}
 

src/Elastic.Apm/Model/Transaction.cs

@@ -347,8 +347,10 @@ private void CheckAndCaptureBaggage()
 			if (!WildcardMatcher.IsAnyMatch(Configuration.BaggageToAttach, baggage.Key))
 				continue;
 
-			Otel ??= new OTel() { Attributes = new Dictionary<string, object>() };
-			Otel.Attributes.Add(baggage.Key, baggage.Value);
+			Otel ??= new OTel { Attributes = new Dictionary<string, object>() };
+
+			var newKey = $"baggage.{baggage.Key}";
+			Otel.Attributes[newKey] = baggage.Value;
 		}
 	}