ExpositionFormats should be optional

[turneand]

Our applications are deployed and make use of only the text based prometheus writers.

Is it possible to make the non-text based writers in ExpositionFormats optional (protobuf and openMetrics), and ideally flagged as optional dependencies? Protobuf binaries add on ~2mb to our deployable which we will never use, and also reduce the risk of any potential transitive vulnerabilities.