Add prometheus timestamp example to readme

[sjwang90]

Based off previous issue: https://github.com/influxdata/telegraf/issues/11283#issuecomment-1155722659

• [ ] Add prometheus timestamp example to: https://github.com/influxdata/telegraf/blob/master/plugins/inputs/prometheus/README.md#source
• [ ] Add test case to: https://github.com/influxdata/telegraf/blob/master/plugins/inputs/prometheus/parser_test.go

[reimda]

There is already a parser unit test that shows that timestamps are in milliseconds. It isn't in the unit tests for the input plugin, but in the parser plugin.

https://github.com/influxdata/telegraf/blob/379b8ea04e8df8017611316cb9069cdf8f1512b9/plugins/parsers/prometheus/parser_test.go#L317

I've updated the issue description to show that this is done and we only need to add an example to the readme for this issue.

[sjwang90]

@reimda do you think you can do add the example to the readme?