Fix: allow getting API token from environment

Allow the API token to be obtained from the environment variable
SM_AGENT_API_TOKEN. If it's passed on the command line, that value takes
precedence. For backwards compatibility, in case the user decides to NOT
replace the existing configuration files (in Debian these are config
files, and the user has the option to NOT replace them during an upgrade
if they have edited them, and because of the way we have set this up,
they *have* edited them).

In the shipped systemd service file, replace passing the token on the
command line in favor of passing it on the environment.

Signed-off-by: Marcelo E. Magallon <marcelo.magallon@grafana.com>

cmd/synthetic-monitoring-agent/main.go

@@ -62,6 +62,14 @@ func run(args []string, stdout io.Writer) error {
 		return nil
 	}
 
+	// If the token is provided on the command line, prefer that. Otherwise
+	// pull it from the environment variable SM_AGENT_API_TOKEN. If that's
+	// not available, fallback to API_TOKEN, which was the environment
+	// variable name previously used in the systemd unit files.
+	//
+	// Using API_TOKEN should be deprecated after March 1st, 2023.
+	*apiToken = stringFromEnv("API_TOKEN", stringFromEnv("SM_AGENT_API_TOKEN", *apiToken))
+
 	if *apiToken == "" {
 		return fmt.Errorf("invalid API token")
 	}
@@ -235,3 +243,11 @@ func newConnectionBackoff() *backoff.Backoff {
 		Jitter: true,
 	}
 }
+
+func stringFromEnv(name string, override string) string {
+	if override != "" {
+		return override
+	}
+
+	return os.Getenv(name)
+}

scripts/package/config/systemd/synthetic-monitoring-agent.conf

@@ -1,5 +1,5 @@
 # Enter API token retrieved from grafana.com here
-API_TOKEN='YOUR TOKEN HERE'
+SM_AGENT_API_TOKEN='YOUR TOKEN HERE'
 
 # Backend API server to connect to
 API_SERVER='synthetic-monitoring-grpc.grafana.net:443' 

scripts/package/config/systemd/synthetic-monitoring-agent.service

@@ -6,7 +6,7 @@ Type=simple
 User=root
 Group=root
 EnvironmentFile=/etc/synthetic-monitoring/synthetic-monitoring-agent.conf
-ExecStart=/usr/bin/synthetic-monitoring-agent --api-token=${API_TOKEN} --api-server-address=${API_SERVER} --api-insecure=${API_INSECURE} --listen-address=${LISTEN_ADDRESS} --verbose=${VERBOSE} --debug=${DEBUG} --features=${FEATURES}
+ExecStart=/usr/bin/synthetic-monitoring-agent --api-server-address=${API_SERVER} --api-insecure=${API_INSECURE} --listen-address=${LISTEN_ADDRESS} --verbose=${VERBOSE} --debug=${DEBUG} --features=${FEATURES}
 Restart=always
 
 [Install]