Expose trace sampling controls in the public API

api/base.proto

@@ -227,3 +227,11 @@ message TransportSocket {
   // See the supported transport socket implementations for further documentation.
   google.protobuf.Struct config = 2;
 }
+
+// Percent, typically used to specify things like target sampling percentages among tracing requests
+// (as in, e.g., :ref:`HTTP Connection Manager tracing
+// <envoy_api_field_filter.network.HttpConnectionManager.tracing>`).
+message Percent {
+  // The percent, a float between 0 and 1.
+  float value = 1 [(validate.rules).float = {gte: 0, lte: 1}];
+}

api/filter/network/http_connection_manager.proto

@@ -80,6 +80,23 @@ message HttpConnectionManager {
     // populate the tag name, and the header value is used to populate the tag value. The tag is
     // created if the specified header name is present in the request's headers.
     repeated string request_headers_for_tags = 2;
+
+    // Global target percentage of requests that will be force traced if the *x-client-trace-id*
+    // header is set. Percent is resolved to the nearest 1% (rounded down). Defaults to 1 (i.e., 100%). This
+    // variable is a direct analog for the variable of the same name in the :ref:`HTTP Connection
+    // Manager <config_http_conn_man_runtime>`.
+    Percent client_enabled = 3;
+
+    // Global target percentage of requests that will be traced after all other checks have been
+    // applied (force tracing, sampling, etc.). Percent is resolved to the nearest 1% (rounded
+    // down). Defaults to 1 (i.e., 100%). This variable is a direct analog for the variable of the same name in
+    // the :ref:`HTTP Connection Manager <config_http_conn_man_runtime>`.
+    Percent global_enabled = 4;
+
+    // Global target percentage of requests that will be randomly traced. Percent is resolved to the
+    // nearest 0.01%, rounded down. Defaults to 1 (i.e., 100%). This variable is a direct analog for
+    // the variable of the same name in the :ref:`HTTP Connection Manager <config_http_conn_man_runtime>`.
+    Percent random_sampling = 5;
   }
 
   // Presence of the object defines whether the connection manager