Add metadata into single sweep (quantumlib#6869)

cirq-google/cirq_google/api/v2/run_context.proto

@@ -126,6 +126,19 @@ message DeviceParameter {
     // by the sweep values themselves.
 }
 
+message Metadata {
+    // Optional arguments for if this is a device parameter.
+    // Note one single_sweep may be associated with multiple device parameters.
+    repeated DeviceParameter device_parameters = 1;
+
+    // If specified, use this label instead of parameter_key as the independent
+    // column name in returned dataset.
+    optional string label = 2;
+
+    // If true, store this sweep as parameters instead of the independent axes.
+    optional bool as_parameter = 3;
+}
+
 // A bundle of multiple DeviceParameters and their values.
 // The main use case is to set those parameters with the
 // values from this bundle before executing a circuit sweep.
@@ -203,6 +216,9 @@ message SingleSweep {
   // Optional arguments for if this is a device parameter.
   // (as opposed to a circuit symbol)
   DeviceParameter parameter = 4;
+
+  // Optional arguments for storing extra metadata information.
+  Metadata metadata = 6;
 }