Adding optional prefixes and labels to `ProcessCollector` metrics via custom registry initialization

[glitchy]

Is your feature request related to a problem? Please describe.

• being a global static, there is not a way for the process metrics created by enabling the process feature flag to acquire optional/dynamic prefix/labels

Describe the solution you'd like

• extend the ProcessCollector the ability to take optional prefix/labels, allowing the user to implement/manage their own

Describe alternatives you've considered

• iterate through all the gathered metric families and metrics and add my labels (too much overhead)
• making my service a scrape target (too much overhead)

Additional context

• there are some use cases where this is warranted, such a distributed compute platform where incurring the extra overhead from the prometheus scrape interval for every worker node is unnecessary when the worker heartbeats/metrics are already gathered at a single point elsewhere (eg. scheduler node) that can be scraped and where the resource overhead is preferred.

[karlem]

Hey! Big +1 for this feature—definitely see the need here. One possible alternative could be adding a separate feature flag, like process-custom, which would export the process_collector without automatically registering it in the default registry. This way, it could be registered manually as needed.