Make special handling of check metrics in StatsD output configurable

[olegbespalov]

The statsd output (also used for NewRelic and DataDog, among others) currently has special handling of check metric samples:

https://github.com/grafana/k6/blob/59a8883676edf4e28edb39561c33ebd941e05944/output/statsd/output.go#L79-L87
https://github.com/grafana/k6/blob/59a8883676edf4e28edb39561c33ebd941e05944/output/statsd/output.go#L94-L100

It transforms the metric names to check.<check-name>.pass or check.<check-name>.fail. I don't remember why this was done, probably because StatsD doesn't seem to support anything like our Rate metric (so we transform it to 2 Count ones here) and because we didn't want the checks to be grouped together (sort of a localized grafana/k6#1321 workaround). In any case, it definitely needs to be better documented (grafana/k6-docs#603) and maybe ideally also configurable (this issue) 🤔

For example, we can add a new K6_STATSD_EXPAND_RATE_ON_TAGS option to the statsd output, with the default value of check. Then:

• the current behavior of the output will be preserved with the default value
• if the user sets K6_STATSD_EXPAND_RATE_ON_TAGS to an empty string, the special handling of checks will be disabled
• if the user sets K6_STATSD_EXPAND_RATE_ON_TAGS to something else, other Rate metrics could be expanded in a similar manner as well

Connected issues and forum threads:

• https://community.k6.io/t/check-function-is-not-sending-metrics/3334
• Datadog integration: send the checks metric. grafana/k6#1403

The issue ported from grafana/k6#2470