Allow to "enable percentiles" from the OpenMetrics configuration

[giuscri]

SSIA. Right now to enable percentiles on OpenMetrics scraped metrics, one must enable that manually from the UI one by one.

[Sandhya58]

Hi @giuscri , I’d like to work on this issue.

I’m planning to add an OpenMetrics config option to enable percentiles from configuration, wire it through the scraper/config path, and add tests/docs. Please let me know if there’s a preferred config name or implementation direction before I start.