feat(client): prepend custom user-agent via FASTLY_USER_AGENT env var…

…iable to user (#531)

* Added FASTLY_USER_AGENT env variable

* Prepend to User-Agent instead of overwrite

fastly/client.go

@@ -54,6 +54,10 @@ const DefaultRealtimeStatsEndpoint = "https://rt.fastly.com"
 // JSONMimeType is the MIME type for the JSON data format.
 const JSONMimeType = "application/json"
 
+// UserAgentEnvVar is the name of an environment variable that can be used
+// to change the User-Agent of the http requests
+const UserAgentEnvVar = "FASTLY_USER_AGENT"
+
 // ProjectURL is the url for this library.
 var ProjectURL = "github.com/fastly/go-fastly"
 
@@ -128,6 +132,10 @@ func NewClientForEndpoint(key, endpoint string) (*Client, error) {
 		client.DebugMode = true
 	}
 
+	if customUserAgent, ok := os.LookupEnv(UserAgentEnvVar); ok {
+		UserAgent = fmt.Sprintf("%s, %s", customUserAgent, UserAgent)
+	}
+
 	return client.init()
 }