Reduce allocation in ExposureApi by serializing to stream

tracer/src/Datadog.Trace/FeatureFlags/Exposure/ExposureApi.cs

@@ -17,6 +17,7 @@
 using Datadog.Trace.HttpOverStreams;
 using Datadog.Trace.Logging;
 using Datadog.Trace.Vendors.Newtonsoft.Json;
+using Datadog.Trace.Vendors.Newtonsoft.Json.Serialization;
 
 namespace Datadog.Trace.FeatureFlags.Exposure;
 
@@ -26,6 +27,15 @@ internal sealed class ExposureApi : IDisposable
 
     private const int DefaultCapacity = 1 << 16; // 65536 elements
     public const string ExposurePath = "evp_proxy/v2/api/v2/exposure";
+    private static readonly JsonSerializerSettings SerializerSettings = new()
+    {
+        NullValueHandling = NullValueHandling.Include,
+        ContractResolver = new DefaultContractResolver
+        {
+            NamingStrategy = new SnakeCaseNamingStrategy(),
+        }
+    };
+
     private readonly TaskCompletionSource<bool> _processExit = new();
     private readonly TimeSpan _sendInterval = TimeSpan.FromSeconds(10);
     private readonly Queue<ExposureEvent> _exposures = new Queue<ExposureEvent>();
@@ -106,10 +116,10 @@ private async Task SendLoopAsync()
                 var apiRequestFactory = _apiRequestFactory;
                 var uri = apiRequestFactory.GetEndpoint(ExposurePath);
                 var payload = TryGetPayload();
-                if (payload.Count != 0)
+                if (payload is not null)
                 {
                     var request = apiRequestFactory.Create(uri);
-                    using var response = await request.PostAsync(payload, MimeTypes.Json).ConfigureAwait(false);
+                    using var response = await request.PostAsJsonAsync(payload, MultipartCompression.None, SerializerSettings).ConfigureAwait(false);
                 }
             }
             catch (Exception ex)
@@ -130,24 +140,22 @@ private async Task SendLoopAsync()
         }
     }
 
-    private ArraySegment<byte> TryGetPayload()
+    private ExposuresRequest? TryGetPayload()
     {
         List<ExposureEvent> exposures;
         lock (_exposures)
         {
             if (_exposures.Count == 0)
             {
                 // nothing to do, skip send
-                return default;
+                return null;
             }
 
             exposures = [.. _exposures];
             _exposures.Clear();
         }
 
-        var request = new ExposuresRequest(_context, exposures);
-        var json = JsonConvert.SerializeObject(request);
-        return new ArraySegment<byte>(Encoding.UTF8.GetBytes(json));
+        return new ExposuresRequest(_context, exposures);
     }
 
     public void SendExposure(in ExposureEvent exposure)