GetCapacityResponse: minimum volume size

Using CapacityRange in GetCapacityResponse is not a good fit because
the names of its fields is a bit awkward. By adding two fields in
GetCapacityResponse we support the same functionality, plus they can
be left unset separately from each other.

Author: pohly

csi.proto

@@ -948,19 +948,41 @@ message GetCapacityResponse {
   // The value of this field MUST NOT be negative.
   int64 available_capacity = 1;
 
-  // The supported volume size that may be used in a CreateVolume call
+  // The largest size that may be used in a
+  // CreateVolumeRequest.capacity_range.required_bytes field
   // to create a volume with the same parameters as those in
-  // GetCapacityRequest, given as a lower bound (`required_bytes`)
-  // and an upper bound (`limit_bytes`).
+  // GetCapacityRequest.
   //
   // If `volume_capabilities` or `parameters` is
   // specified in the request, the Plugin SHALL take those into
-  // consideration when calculating the supported volume size of the
+  // consideration when calculating the minimum volume size of the
   // storage.
   //
-  // This field is OPTIONAL. COs MAY use it to make decision about
+  // This field is OPTIONAL. MUST NOT be negative.
+  // The Plugin SHOULD provide a value for this field if it has
+  // a maximum size for individual volumes and leave it unset
+  // otherwise. COs MAY use it to make decision about
   // where to create volumes.
-  CapacityRange volume_size = 2 [(alpha_field) = true];
+  google.protobuf.Int64Value maximum_volume_size = 2
+    [(alpha_field) = true];
+
+  // The smallest size that may be used in a
+  // CreateVolumeRequest.capacity_range.limit_bytes field
+  // to create a volume with the same parameters as those in
+  // GetCapacityRequest.
+  //
+  // If `volume_capabilities` or `parameters` is
+  // specified in the request, the Plugin SHALL take those into
+  // consideration when calculating the maximum volume size of the
+  // storage.
+  //
+  // This field is OPTIONAL. MUST NOT be negative.
+  // The Plugin SHOULD provide a value for this field if it has
+  // a minimum size for individual volumes and leave it unset
+  // otherwise. COs MAY use it to make decision about
+  // where to create volumes.
+  google.protobuf.Int64Value minimum_volume_size = 3
+    [(alpha_field) = true];
 }
 message ControllerGetCapabilitiesRequest {
   // Intentionally empty.