In v2, we used google.protobuf.Empty for the configuration for simple extensions that didn't require custom configuration. Now that we have Any as the canonical method for specifying extensions, this doesn't work as well, since it makes it difficult to move to a versioned per-extension message without breaking wire compatibility within a major version.

As part of the v3 works, we should give each existing filter that relies on google.protobuf.Empty a custom empty config message in v3 and deprecate the use of WKT here.