Read credentials from env variable if not set

Allows for secrets to be provided to exporter outside of main
configuration. E.G. via Kubernetes Secrets.
Specified configuration files or flags take precedence over environment
variables.

README.md

@@ -35,6 +35,14 @@ where `address` is the address of your router. `device` is the label name for th
 in the metrics output to prometheus. The `user` and `password` are the ones you
 created for the exporter to use to access the API.
 
+User and password flags can be set with the `MIKROTIK_USER` and `MIKROTIK_PASSWORD` environment variables, respectively.
+
+```
+MIKROTIK_USER=prometheus
+MIKROTIK_PASSWORD=changeme
+./mikrotik-exporter -address 10.10.0.1 -device my_router
+```
+
 #### Config File
 
 `./mikrotik-exporter -config-file config.yml`

main.go

@@ -115,6 +115,13 @@ func loadConfigFromFile() (*config.Config, error) {
 }
 
 func loadConfigFromFlags() (*config.Config, error) {
+	// Attempt to read credentials from env if not already defined
+	if *user == "" {
+		*user = os.Getenv("MIKROTIK_USER")
+	}
+	if *password == "" {
+		*password = os.Getenv("MIKROTIK_PASSWORD")
+	}
 	if *device == "" || *address == "" || *user == "" || *password == "" {
 		return nil, fmt.Errorf("missing required param for single device configuration")
 	}