Consume app_id on app registration + pass /w user-agent on request

- Let's ensure that we know which app made a given request by encoding within
  the user-agent
- This will help /w ascertaining who is making a given request

lib/client.js

@@ -557,7 +557,8 @@ export default class Client {
         options = { url: options }
       }
 
-      options.headers = addUserAgent(options.headers)
+      const appId = this._metadata && this._metadata.appId
+      options.headers = addUserAgent(options.headers, appId)
 
       const doneEventKey = `${requestKey}.done`
       const failEventKey = `${requestKey}.fail`

lib/utils.js

@@ -3,11 +3,13 @@ import pkgJson from '../package.json'
 
 const Promise = window.Promise || NativePromise
 
-export function addUserAgent (headers) {
+export function addUserAgent (headers, appId) {
   const originalUserAgent = headers && headers['User-Agent'] ? headers['User-Agent'] : ''
+  const zafSdkUserAgentString = `zendesk_app_framework_sdk/sdk_version:${pkgJson.version}/app_id:${appId}`
+  const userAgent = originalUserAgent ? `${originalUserAgent} ${zafSdkUserAgentString}` : zafSdkUserAgentString
   return {
     ...headers,
-    'User-Agent': originalUserAgent ? `${originalUserAgent} zendesk_app_framework_sdk/${pkgJson.version}` : `zendesk_app_framework_sdk/${pkgJson.version}`
+    'User-Agent': userAgent
   }
 }