Add some notes on Retry-After and ETag

src/_til/2024/2024-02-28-get-netlify-usage-from-api.md

@@ -1,6 +1,7 @@
 ---
 layout: til
 date: 2024-02-28 10:15:24 +0000
+date_updated: 2024-06-12 10:58:07 +0100
 title: Get my Netlify bandwidth usage from the API
 tags:
   - netlify
@@ -43,3 +44,34 @@ A couple of observations on the data you get back:
 
 *   This value gets cached for a while -- in the example above, I called the API at 15:22 but the `last_updated_at` value is 15:03.
     I'm not sure how often the value is updated.
+
+## Being a responsible user of the API
+
+*   The API returns a `Retry-After` value, which is a minute after your request.
+    This is a clue about how often the value gets updated -- it's definitely not second-level granularity.
+
+    ```console
+    $ curl -v -H 'Authorization: Bearer <TOKEN>' 'https://api.netlify.com/api/v1/accounts/<team_slug>/bandwidth'
+    …
+    < HTTP/2 200
+    < date: Wed, 12 Jun 2024 09:52:37 GMT
+    …
+    < retry-after: 2024-06-12 09:53:37 UTC
+    …
+    ```
+
+*   The responses include an `ETag` header, which you can use to check if the value has changed since the last request:
+
+    ```console
+    $ curl -v -H 'Authorization: Bearer <TOKEN>'  'https://api.netlify.com/api/v1/accounts/<TEAM_SLUG>/bandwidth'
+    …
+    < HTTP/2 200
+    < etag: W/"190b2e3e6d8c239bd546819b0f0b1eb9"
+    …
+
+    $ curl -v  -H 'If-None-Match: W/"190b2e3e6d8c239bd546819b0f0b1eb9"' -H 'Authorization: Bearer <TOKEN>' 'https://api.netlify.com/api/v1/accounts/<TEAM_SLUG>/bandwidth'
+    < HTTP/2 304
+    …
+    ```
+    
+    Annoyingly, this doesn't make the API any faster -- responses still take about half a second, even if nothing has changed.