Update and improved instructions to fix API restrictions problem for FIS (#1425)

Updated according to Stackoverflow question and latest MSA / developer outreach emails.

Author: andirayo

firebase-installations/API_KEY_RESTRICTIONS.md

@@ -13,19 +13,20 @@ The following SDKs updates introduce a dependency on the [Firebase Installations
 - ML Kit
 - Instance ID
 
-
-As a result, API restrictions you may have applied to API keys used by your Firebase applications may have to be updated to allow your apps to call the Firebase Installations API.
+As a result, API restrictions you may have applied to API keys used by your Firebase applications may have to be updated to allow your apps to call the [Firebase Installations API](https://console.cloud.google.com/apis/library/firebaseinstallations.googleapis.com).
 
 ## What do I need to do?
 
 Before upgrading your application(s) to the latest SDK version, please **make sure that the API key(s) used in your application(s) are whitelisted for the Firebase Installations API:**
 
-- **Open** the [Google Cloud Platform Console](https://console.cloud.google.com/apis/credentials?folder).
-- **Choose** the project you use for your application(s).
-- **Open**  `APIs & Services` and **select** `Credentials`.
-- **Click** `Edit API Key` (pencil icon) for the API key in question.
-- **Scroll down** to the `API restrictions` section.
-- If the radio button shows `Don't restrict key`, the API key is not affected.
-Otherwise, from the dropdown menu, **add** the `Firebase Installations API` to the list of permitted APIs, and click `Save`.
-
-**Note:** If you cannot find the [Firebase Installations API](https://console.cloud.google.com/apis/library/firebaseinstallations.googleapis.com) in the list of APIs, you might first have to enable the API for your project (to do so [click here](https://console.cloud.google.com/apis/library/firebaseinstallations.googleapis.com)).
+1. **Open** the [Google Cloud Platform Console](https://console.cloud.google.com/apis/credentials?folder).
+1. **Choose** the project you use for your application(s).
+1. **Open**  `APIs & Services` and **select** `Credentials`.
+1. **Click** `Edit API Key` (pencil icon) for the API key in question.
+1. **Scroll down** to the `API restrictions` section.
+1. From the dropdown menu, **add** the `Firebase Installations API` to the list of permitted APIs, and click `Save`.
+1. If the radio button shows `Don't restrict key`, you may be looking at the wrong API key. \
+   You can check which API key is used for the Firebase Installations API by looking at the [service usage page for your project](https://console.cloud.google.com/apis/api/firebaseinstallations.googleapis.com/credentials).
+
+**Note**: **Verify** your fix by checking if you can see successful `200` requests increasing on the [Firebase Installations API request metrics page](https://console.cloud.google.com/apis/api/firebaseinstallations.googleapis.com/metrics). \
+**Note**: If you cannot find the Firebase Installations API in the list of APIs, you might first have to enable the API for your project (to do so [click here](https://console.cloud.google.com/apis/library/firebaseinstallations.googleapis.com)).