Initial implementation of OpenMetrics text format

[qoega]

Added support of OpenMetrics(Prometheus compatible) text format. Spec: https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#text-format

Important details:

• Different fieldValues are flattened to separate rows.
• Empty value is not supported, so nil fieldValues are not exported.

As OpenMetrics is a common spec I think load should not test only one system, but it is possible to test OpenMetrics data load to different systems that support this ingestion format. So it kind of adds another dimension to harness.

[CLAassistant]

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[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[qoega]

Can we sign Corporate CLA instead?