AIP-203(optional-consistency) conflicting with AIP-154

[joehan]

Recently, while implementing AIP-154 for an existing resource, I ran into a minor issue with the linter.

AIP-154 specifies that the etag field should not be given any behavior annotations, so I omitted them. However, the resource already had '[(google.api.field_behavior) = OPTIONAL]' specified on some fields, so the AIP-203(optional-consistency) rule triggered.

This was simple enough to address by adding a disabling comment, but I wanted to flag this in case the linter can be refined to not trigger here.