AIP-154: etag field_behavior

[loeffel-io]

AIP-154 mentions

Etags on request methods

On a request message, the etag field should be given a behavior annotation - either REQUIRED or OPTIONAL. See AIP-203 for more information.

But there is a linter error when using REQUIRED:

- file_path: mindful/earth/user/v1/user.proto
  problems:
    - message: Delete RPCs must only require fields explicitly described in AIPs, not "etag".
      location:
        start_position:
            line_number: 282
            column_number: 3
        end_position:
            line_number: 282
            column_number: 59
        path: mindful/earth/user/v1/user.proto
      rule_id: core::0135::request-required-fields
      rule_doc_uri: https://linter.aip.dev/135/request-required-fields

message:

message DeleteUserRequest {
  // The resource name
  // Format: users/{user}
  string name = 1 [
    (google.api.resource_reference) = {type: "user.mindful.com/User"},
    (google.api.field_behavior) = REQUIRED,
    (validate.rules).string = {pattern: "^users\\/[a-f0-9]{8}-[a-f0-9]{4}-[a-f0-9]{4}-[a-f0-9]{4}-[a-f0-9]{12}"}
  ];
  // The etag of the user. Must match the server's etag.
  string etag = 2 [(google.api.field_behavior) = REQUIRED];
}