ProfileEventTrackedProperties.metric no longer required; new enum value added

Two changes in this diff:

• ProfileEventTrackedProperties.metric is removed from the required array and gains nullable: true with a default of "". Clients that previously relied on metric always being present in responses may break; servers that previously required metric in requests no longer do, which is additive for request senders but potentially breaking for consumers expecting it.
• A new enum value "full" is added to a size/image-related field's enum (appears to be an image size selector), which is a purely additive change for that field.