local check to retrieve metrics from a Caddy web server

The check queries Caddy's built-in Promethesus metrics endpoint and derives the following metrics from it:

• number of currently active connections (ongoing requests)
• total number of accepted connections (completed requests)
• error rate (number of requests that failed over a period)
• requests per second
• average request response time
• data transfer rate (responses only, meaning speed of outgoing traffic over a period)
• request transfer rate (data transferred per response over a period)

See web site for setup instructions.