Add option to control simultaneous scrape behavior.

[higecko]

Current behavior during simultaneous scrapes is to proceed at the same time. This might not be the desired behavior. The options should be:

• Proceed with simultaneous scrapes.
• Do not collect Hangfire statistics or update metrics, but don't fail the scrape.
• Fail the scrape.

Default should be the current behavior: to proceed with simultaneous scrapes.