Consider specify metrics version for opinionated_metrics

[Banyc]

The latest metrics as of now is 0.21.1, which is not the same as the version opinionated_metrics depends on (that is 0.20.1). The static RECORDERs of the two metrics versions are different instances. If users added the wrong metrics version, counter!() will be sent to the wrong uninitialized RECORDER, instead of being collected by the initiated one.

It will be great if the docs explicitly tell users which metrics version to use.