`prometheus_metric_version` is not documented in prometheus parser docs

[snophey]

The documentation for the prometheus parser does not mention that there are two different versions of the parser that would parse the same metric differently. This leads to confusion, especially since the current README.md states that the parser does not support any additional configuration options. Meanwhile, there does exist the option prometheus_metric_version which is currently not documented and allows switching between v1 and v2 of the parser.