Support open metrics format as output

[aparcar]

I'm using GitLab and perform benchmarks on multiple rust tools. GitLab support the openmetrics format so it would be useful if it would be directly supported as a cargo-criterion output format.

https://openmetrics.io https://docs.gitlab.com/ee/ci/testing/metrics_reports.html

[aparcar]

As a workaround it is possible to use the following jq command

jq --raw-output '. | select(.id != null)  | {"id": .id | sub("[ /]"; "_"; "g"), "estimate": .mean["estimate"]} | join("_ns ")'

[bheisler]

Hello! Thanks for the suggestion.

This does seem like a useful option. I don't think this will be a high priority for me to implement myself, but a pull request would be welcome.