`prometheus_metric_version` is not documented in prometheus parser docs

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](https://github.com/influxdata/telegraf/tree/master/plugins/parsers/prometheus) 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.

Start with plugins/parsers/prometheus/README.md, linked in the issue. Document that the parser has v1 and v2 behavior and describe the prometheus_metric_version option, including that it selects between them. Done means the README no longer says the parser has no additional configuration options and clearly explains this setting.

Written by the indexing model from the issue text.