Rename config key from DD_TRACE_DISCOVERY_ENABLED to DD_DISCOVERY_SERVICE_ENABLED

Change environment variable name to be more consistent with Datadog naming conventions.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: lucaspimentel

tracer/src/Datadog.Trace/Configuration/ConfigurationKeys.cs

@@ -591,7 +591,7 @@ internal static partial class ConfigurationKeys
         /// This is useful in environments where the discovery endpoint is not available (e.g., Azure Functions with Rust agent).
         /// Default value is true (discovery service enabled).
         /// </summary>
-        public const string DiscoveryServiceEnabled = "DD_TRACE_DISCOVERY_ENABLED";
+        public const string DiscoveryServiceEnabled = "DD_DISCOVERY_SERVICE_ENABLED";
 
         /// <summary>
         /// String constants for CI Visibility configuration keys.