Werk #14562: Prometheus: allow specifying a path prefix

Previously, the Prometheus special agent would use the path prefix as a URL base prefix. For example, specifying the use of the host IP address, e.g. 1.2.3.4, together with a prefix, e.g., my_prefix would result in an API call to

https://my_prefix1.2.3.4/api/v1/status/buildinfo

In particular, this behaviour deviated from the description in the Prometheus rule. With this werk, the path prefix is prepended to the API calls instead, e.g.,

https://1.2.3.4/my_prefix/api/v1/status/buildinfo

Users, which have specified a path prefix, are migrated to the new option Custom URL base prefix, which has the same behaviour as the old path prefix option.

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