tracing: add datadog extension (#4699)

Signed-off-by: Caleb Gilmour <caleb.gilmour@datadoghq.com>

Mirrored from https://github.com/envoyproxy/envoy @ 46c069314913d180c85eaf2f38ce7c1b987e3a81

envoy/config/trace/v2/trace.proto

@@ -23,15 +23,21 @@ import "validate/validate.proto";
 message Tracing {
   message Http {
     // The name of the HTTP trace driver to instantiate. The name must match a
-    // supported HTTP trace driver. *envoy.lightstep*, *envoy.zipkin*, and
-    // *envoy.dynamic.ot* are built-in trace drivers.
+    // supported HTTP trace driver. Built-in trace drivers:
+    //
+    // - *envoy.lightstep*
+    // - *envoy.zipkin*
+    // - *envoy.dynamic.ot*
+    // - *envoy.tracers.datadog*
     string name = 1 [(validate.rules).string.min_bytes = 1];
 
     // Trace driver specific configuration which depends on the driver being instantiated.
-    // See the :ref:`LightstepConfig
-    // <envoy_api_msg_config.trace.v2.LightstepConfig>`, :ref:`ZipkinConfig
-    // <envoy_api_msg_config.trace.v2.ZipkinConfig>`, and :ref:`DynamicOtConfig
-    // <envoy_api_msg_config.trace.v2.DynamicOtConfig>` trace drivers for examples.
+    // See the trace drivers for examples:
+    //
+    // - :ref:`LightstepConfig <envoy_api_msg_config.trace.v2.LightstepConfig>`
+    // - :ref:`ZipkinConfig <envoy_api_msg_config.trace.v2.ZipkinConfig>`
+    // - :ref:`DynamicOtConfig <envoy_api_msg_config.trace.v2.DynamicOtConfig>`
+    // - :ref:`DatadogConfig <envoy_api_msg_config.trace.v2.DatadogConfig>`
     oneof config_type {
       google.protobuf.Struct config = 2;
 
@@ -86,6 +92,14 @@ message DynamicOtConfig {
   google.protobuf.Struct config = 2;
 }
 
+// Configuration for the Datadog tracer.
+message DatadogConfig {
+  // The cluster to use for submitting traces to the Datadog agent.
+  string collector_cluster = 1 [(validate.rules).string.min_bytes = 1];
+  // The name used for the service when traces are generated by envoy.
+  string service_name = 2 [(validate.rules).string.min_bytes = 1];
+}
+
 // Configuration structure.
 message TraceServiceConfig {
   // The upstream gRPC cluster that hosts the metrics service.