Add basic Prometheus metrics handler (#1830)

* feat: Add Prometheus configuration options

* feat: Add Prometheus metrics handler

* build: prometheus became direct dependency

* docs: change description for prometheus metrics path

conf/configuration.go

@@ -57,6 +57,7 @@ type configOptions struct {
 	PasswordEncryptionKey  string
 	ReverseProxyUserHeader string
 	ReverseProxyWhitelist  string
+	Prometheus             prometheusOptions
 
 	Scanner scannerOptions
 
@@ -101,6 +102,11 @@ type listenBrainzOptions struct {
 	BaseURL string
 }
 
+type prometheusOptions struct {
+	Enabled     bool
+	MetricsPath string
+}
+
 var (
 	Server = &configOptions{}
 	hooks  []func()
@@ -244,6 +250,9 @@ func init() {
 	viper.SetDefault("reverseproxyuserheader", "Remote-User")
 	viper.SetDefault("reverseproxywhitelist", "")
 
+	viper.SetDefault("prometheus.enabled", false)
+	viper.SetDefault("prometheus.metricspath", "/metrics")
+
 	viper.SetDefault("scanner.extractor", consts.DefaultScannerExtractor)
 	viper.SetDefault("scanner.genreseparators", ";/,")