add container ID header to MinimalAgentHeaderHelper

Author: vandonr

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

@@ -415,7 +415,7 @@ private static IApiRequestFactory CreateApiRequestFactory(ExporterSettings expor
                 productName: "discovery",
                 tcpTimeout: tcpTimeout,
                 BuildHeaders(containerId),
-                () => new MinimalAgentHeaderHelper(),
+                () => new MinimalAgentHeaderHelper(containerId),
                 uri => uri);
         }
     }

tracer/src/Datadog.Trace/HttpOverStreams/MinimalAgentHeaderHelper.cs

@@ -11,6 +11,13 @@ namespace Datadog.Trace.HttpOverStreams;
 internal class MinimalAgentHeaderHelper : HttpHeaderHelperBase
 {
     private static string? _metadataHeaders = null;
+    private static string? _metadataHeadersWithContainerId = null;
+    private readonly string? _containerId;
+
+    public MinimalAgentHeaderHelper(string? containerId = null)
+    {
+        _containerId = containerId;
+    }
 
     protected override string MetadataHeaders
     {
@@ -22,6 +29,18 @@ protected override string MetadataHeaders
                 _metadataHeaders = string.Concat(headers);
             }
 
+            if (_containerId != null)
+            {
+                if (_metadataHeadersWithContainerId == null)
+                {
+                    // Assuming we'd always get the same container ID. The first time, we use its value to build the header,
+                    // then it's just a marker that we want the header with the containerID in it, and we don't look at its value.
+                    _metadataHeadersWithContainerId = _metadataHeaders + $"{AgentHttpHeaderNames.ContainerId}: {_containerId}{DatadogHttpValues.CrLf}";
+                }
+
+                return _metadataHeadersWithContainerId;
+            }
+
             return _metadataHeaders;
         }
     }