Add Prometheus exporter

Makefile

cli/daemon.go

config/options.go

config/parser.go

go.mod

go.sum

metric/metric.go

miniflux.1

reader/feed/handler.go

reader/processor/processor.go

reader/scraper/scraper.go

reader/scraper/scraper_test.go

service/httpd/httpd.go

service/scheduler/scheduler.go

storage/entry.go

storage/feed.go

storage/user.go

ui/about.go

ui/api_key_create.go

ui/api_key_list.go