Add flag prometheus.web.config.file

[hienvanhuynh]

Current web.config.file does not use the same format as prometheus web.config.file. It is also lacking http headers and basic authentication handling. This is to add prometheus.web.config.file flag that works just like web.config.file of prometheus. It allows configuring http headers, while keeping the old flag web.config.file for backward compatibility.

[hienvanhuynh]

@jacksontj what do you think? This is useful when we want to set http headers to the response