Metric extends Updatable but its all properties are immutable #987

The old Metric metadata object extends [Updatable interface|https://github.com/gooddata/gooddata-java/blob/db678825ed8f1bbeb34cbd8ef5b38a82a28c9947/gooddata-java-model/src/main/java/com/gooddata/sdk/model/md/Updatable.java#L11], but all its properties are not mutable. This means that even we can call MetadataService#updateObj, we aren't able to update anything.

Allow to update important Metric properties, at least its expression and format.