Add DD_AGENT_FEATURE_POLLING_ENABLED setting (#7616)

## Summary

Adds `DD_AGENT_FEATURE_POLLING_ENABLED` environment variable (default:
`true`) to allow disabling the agent discovery service. When disabled,
the tracer uses `NullDiscoveryService` instead of querying the agent for
available endpoints.

## Motivation

In certain environments, such as Azure Functions with the Rust agent,
the discovery endpoint is not available. Currently, the tracer always
attempts to query the discovery service, which results in unnecessary
network calls and potential errors in these scenarios.

## Changes

- Added `DD_AGENT_FEATURE_POLLING_ENABLED` configuration key to
`ConfigurationKeys.cs`
- Added `AgentFeaturePollingEnabled` property to `TracerSettings`
(defaults to `true`)
- Updated `TracerManagerFactory.GetDiscoveryService()` to return
`NullDiscoveryService.Instance` when disabled
- Changed `GetDiscoveryService()` visibility from `protected` to
`internal virtual` for consistency with override in
`TestOptimizationTracerManagerFactory`
- Added unit test `DiscoveryServiceCanBeDisabled` to verify both enabled
and disabled states

## Test Plan

- [x] Unit test verifies setting is read correctly
- [x] Unit test verifies `DiscoveryService` is returned when enabled
- [x] Unit test verifies `NullDiscoveryService.Instance` is returned
when disabled
- [ ] Manual testing in Azure Functions environment with Rust agent

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

Author: lucaspimentel

tracer/src/Datadog.Trace/Agent/DiscoveryService/DiscoveryService.cs

@@ -17,6 +17,9 @@
 
 namespace Datadog.Trace.Agent.DiscoveryService
 {
+    /// <summary>
+    /// Queries the Datadog Agent and discovers which version we are running against and which endpoints it supports.
+    /// </summary>
     internal class DiscoveryService : IDiscoveryService
     {
         private const string SupportedDebuggerEndpoint = "debugger/v1/input";

tracer/src/Datadog.Trace/Agent/DiscoveryService/IDiscoveryService.cs

@@ -11,7 +11,7 @@
 namespace Datadog.Trace.Agent.DiscoveryService
 {
     /// <summary>
-    /// Queries datadog-agent and discovers which version we are running against and what endpoints it supports.
+    /// Queries the Datadog Agent and discovers which version we are running against and which endpoints it supports.
     /// </summary>
     internal interface IDiscoveryService
     {

tracer/src/Datadog.Trace/Ci/TestOptimizationTracerManagerFactory.cs

@@ -107,7 +107,7 @@ protected override IAgentWriter GetAgentWriter(TracerSettings settings, IDogStat
             return new ApmAgentWriter(settings, updateSampleRates, discoveryService, traceBufferSize);
         }
 
-        protected override IDiscoveryService GetDiscoveryService(TracerSettings settings)
+        internal override IDiscoveryService GetDiscoveryService(TracerSettings settings)
             => _testOptimizationTracerManagement.DiscoveryService;
     }
 }

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

@@ -599,6 +599,13 @@ internal static partial class ConfigurationKeys
         /// </summary>
         public const string ApplicationMonitoringConfigFileEnabled = "DD_APPLICATION_MONITORING_CONFIG_FILE_ENABLED";
 
+        /// <summary>
+        /// Configuration key to disable polling the /info endpoint in the trace agent for feature discovery.
+        /// Default value is true (polling enabled).
+        /// </summary>
+        /// <seealso cref="TracerSettings.AgentFeaturePollingEnabled"/>
+        public const string AgentFeaturePollingEnabled = "DD_AGENT_FEATURE_POLLING_ENABLED";
+
         /// <summary>
         /// String constants for CI Visibility configuration keys.
         /// </summary>

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

@@ -465,6 +465,10 @@ not null when string.Equals(value, "otlp", StringComparison.OrdinalIgnoreCase) =
                                              .WithKeys(ConfigurationKeys.AzureEventHubsBatchLinksEnabled)
                                              .AsBool(defaultValue: true);
 
+            AgentFeaturePollingEnabled = config
+                                        .WithKeys(ConfigurationKeys.AgentFeaturePollingEnabled)
+                                        .AsBool(defaultValue: true);
+
             DelayWcfInstrumentationEnabled = config
                                             .WithKeys(ConfigurationKeys.FeatureFlags.DelayWcfInstrumentationEnabled)
                                             .AsBool(defaultValue: true);
@@ -1008,6 +1012,15 @@ not null when string.Equals(value, "otlp", StringComparison.OrdinalIgnoreCase) =
         /// <seealso cref="ConfigurationKeys.AzureEventHubsBatchLinksEnabled"/>
         public bool AzureEventHubsBatchLinksEnabled { get; }
 
+        /// <summary>
+        /// Gets a value indicating whether the agent discovery service is enabled.
+        /// When disabled, the tracer will not query the agent for available endpoints.
+        /// 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>
+        /// <seealso cref="ConfigurationKeys.AgentFeaturePollingEnabled"/>
+        public bool AgentFeaturePollingEnabled { get; }
+
         /// <summary>
         /// Gets a value indicating whether to enable the updated WCF instrumentation that delays execution
         /// until later in the WCF pipeline when the WCF server exception handling is established.

tracer/src/Datadog.Trace/TracerManagerFactory.cs

@@ -458,9 +458,6 @@ private static string GetUrl(TracerSettings settings)
             }
         }
 
-        protected virtual IDiscoveryService GetDiscoveryService(TracerSettings settings)
-            => DiscoveryService.Create(settings.Exporter);
-
         internal static IDogStatsd CreateDogStatsdClient(TracerSettings settings, string serviceName, List<string> constantTags, string prefix = null, TimeSpan? telemtryFlushInterval = null)
         {
             try
@@ -507,6 +504,12 @@ internal static IDogStatsd CreateDogStatsdClient(TracerSettings settings, string
             }
         }
 
+        // internal for testing
+        internal virtual IDiscoveryService GetDiscoveryService(TracerSettings settings)
+            => settings.AgentFeaturePollingEnabled ?
+                   DiscoveryService.Create(settings.Exporter) :
+                   NullDiscoveryService.Instance;
+
         private static IDogStatsd CreateDogStatsdClient(TracerSettings settings, string serviceName)
         {
             var customTagCount = settings.GlobalTags.Count;

tracer/test/Datadog.Trace.Tests/Configuration/ConfigurationSourceTests.cs

@@ -91,6 +91,7 @@ public ConfigurationSourceTests()
 
             yield return (s => s.TraceId128BitGenerationEnabled, true);
             yield return (s => s.TraceId128BitLoggingEnabled, true);
+            yield return (s => s.AgentFeaturePollingEnabled, true);
         }
 
         public static IEnumerable<(string Key, string Value, Func<TracerSettings, object> Getter, object Expected)> GetBreakingChangeTestData()

tracer/test/Datadog.Trace.Tests/Configuration/TracerSettingsTests.cs

@@ -78,7 +78,7 @@ public void Constructor_HandlesNullSource()
         }
 
         [Fact]
-        public void Constructor_HandlesEmptyource()
+        public void Constructor_HandlesEmptySource()
         {
             var tracerSettings = new TracerSettings(new NameValueConfigurationSource(new()));
             tracerSettings.Should().NotBeNull();

tracer/test/Datadog.Trace.Tests/Telemetry/config_norm_rules.json

@@ -309,6 +309,7 @@
   "dbm_propagation_mode": "dbm_propagation_mode",
   "trace.remove_root_span_laravel_queue": "trace_remove_root_span_laravel_queue_enabled",
   "trace.remove_autoinstrumentation_orphans": "trace_remove_auto_instrumentation_orphans_enabled",
+  "DD_AGENT_FEATURE_POLLING_ENABLED": "agent_feature_polling_enabled",
   "DD_TRACE_AWS_ADD_SPAN_POINTERS": "trace_aws_add_span_pointers",
   "DD_TRACE_CONFIG_FILE": "trace_config_file",
   "DD_DOTNET_TRACER_CONFIG_FILE": "trace_config_file",

tracer/test/Datadog.Trace.Tests/TracerManagerFactoryTests.cs

@@ -103,6 +103,29 @@ public void RemoteConfigIsDisabledInAzureAppServices()
         _manager.TracerFlareManager.Should().BeOfType<NullTracerFlareManager>();
     }
 
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public void DiscoveryServiceCanBeDisabled(bool enabled)
+    {
+        var source = CreateConfigurationSource((ConfigurationKeys.AgentFeaturePollingEnabled, enabled.ToString()));
+        var settings = new TracerSettings(source);
+
+        settings.AgentFeaturePollingEnabled.Should().Be(enabled);
+
+        var factory = new TracerManagerFactory();
+        var discoveryService = factory.GetDiscoveryService(settings);
+
+        if (enabled)
+        {
+            discoveryService.Should().BeOfType<DiscoveryService>();
+        }
+        else
+        {
+            discoveryService.Should().BeSameAs(NullDiscoveryService.Instance);
+        }
+    }
+
     private static TracerManager CreateTracerManager(TracerSettings settings)
     {
         return new TracerManagerFactory().CreateTracerManager(