Limit attribute count and truncate values in ElasticActivityListener (#…

…2461)

Implement limits to comply with OpenTelemetry specs, capping attributes
at 128 and truncating string values to 10,000 characters. This ensures
payload size remains manageable and adheres to the defined standards.

https://opentelemetry.io/docs/specs/otel/common/#attribute-limits

src/Elastic.Apm/OpenTelemetry/ElasticActivityListener.cs

@@ -228,9 +228,20 @@ private static void UpdateOTelAttributes(Activity activity, OTel otel)
 		{
 			if (!activity.TagObjects.Any()) return;
 
+			// https://opentelemetry.io/docs/specs/otel/common/#attribute-limits
+			// copy max 128 keys and truncate values to 10k chars (the current maximum for e.g. statement.db).
+			var i = 0;
 			otel.Attributes ??= new Dictionary<string, object>();
 			foreach (var (key, value) in activity.TagObjects)
-				otel.Attributes[key] = value;
+			{
+				if (i >= 128) break;
+
+				if (value is string s)
+					otel.Attributes[key] = s.Truncate(10_000);
+				else
+					otel.Attributes[key] = value;
+				i++;
+			}
 		}
 
 		private static void UpdateSpan(Activity activity, Span span)