OpenMetrics support

[marcuslindfeldt]

OpenMetrics is aiming to be the standard for exposing metrics data and is already being adopted by many popular projects.

If k6 would expose metrics in this format we would not be limited to using influxDb to store loadtest metrics.

OpenCensus seems like a good library for instrumentation.

[liclac]

I would like to add that OpenMetrics is under the CNCF umbrella as it moves towards a stable spec.

The Prometheus metrics format, which this is based on, is already a de-facto standard.

[juwatanabe]

+1 this feature request

[olegbespalov]

Closing in favor of the #2557