Support OpenMetrics

[ikurovsky]

Extend support for OpenMetrics including appropriate export. Currently there is no way to define some of its domain entries such as types (e.g. info and stateset), there is no support for units.

[rc-mairovichaa]

Also in case of Prometheus format string (Info) exporting is missing.

ps: maybe some other types also are missing, but i know so far only about string.

UPD: I was misinformed. OpenMetrics format doesn't provide possibility to export string, but merely says that actual values of the metric are present in its labels. https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#info