Add the data dog rate limiter to the autoscaler docs (#26130)

* func: add documentation for the data dog rate limiter

* Update datadog.mdx

* Update website/content/tools/autoscaling/plugins/apm/datadog.mdx

Co-authored-by: Aimee Ukasick <aimee.ukasick@hashicorp.com>

* Update website/content/tools/autoscaling/plugins/apm/datadog.mdx

Co-authored-by: Tim Gross <tgross@hashicorp.com>

---------

Co-authored-by: Aimee Ukasick <aimee.ukasick@hashicorp.com>
Co-authored-by: Tim Gross <tgross@hashicorp.com>

website/content/tools/autoscaling/plugins/apm/datadog.mdx

@@ -18,6 +18,7 @@ apm "datadog" {
   config = {
     dd_api_key = "<api key>"
     dd_app_key = "<app key>"
+    dd_api_rate_per_second = "200"
 
     site = "datadoghq.eu"
   }
@@ -30,6 +31,9 @@ apm "datadog" {
 - `dd_app_key` `(string: <required>)` - The Datadog application key to use for
   authentication.
 
+- `dd_api_rate_per_second` `(string: "")` - Maximum rate limit to apply to the 
+  underlying HTTP client used to query the Datadog API.
+
 - `site` `(string: "datadoghq.com")` - The Datadog site to connect to.
 
 The Datadog plugin can also read its configuration options via environment