initial prometheus metrics support, simplfied some code

.env.example

@@ -2,21 +2,24 @@
 TZ=ETC/UTC
 
 # generate secret with `openssl rand -base64 32`
-GOPROXY_API_JWT_SECRET=
+GODOXY_API_JWT_SECRET=
 
 # the JWT token time-to-live
-GOPROXY_API_JWT_TOKEN_TTL=1h
+GODOXY_API_JWT_TOKEN_TTL=1h
 
 # API/WebUI login credentials
-GOPROXY_API_USER=admin
-GOPROXY_API_PASSWORD=password
+GODOXY_API_USER=admin
+GODOXY_API_PASSWORD=password
 
 # Proxy listening address
-GOPROXY_HTTP_ADDR=:80
-GOPROXY_HTTPS_ADDR=:443
+GODOXY_HTTP_ADDR=:80
+GODOXY_HTTPS_ADDR=:443
 
 # API listening address
-GOPROXY_API_ADDR=127.0.0.1:8888
+GODOXY_API_ADDR=127.0.0.1:8888
+
+# Prometheus Metrics listening address (uncomment to enable)
+#GODOXY_PROMETHEUS_ADDR=:8889
 
 # Debug mode
-GOPROXY_DEBUG=false
+GODOXY_DEBUG=false