Document the X-Opaque-Id header (#8820) (#8875)

_api-reference/common-parameters.md

@@ -123,4 +123,17 @@ Kilometers | `km` or `kilometers`
 Meters | `m` or `meters`
 Centimeters | `cm` or `centimeters`
 Millimeters | `mm` or `millimeters`
-Nautical miles | `NM`, `nmi`, or `nauticalmiles` 
+Nautical miles | `NM`, `nmi`, or `nauticalmiles` 
+
+## `X-Opaque-Id` header
+
+You can specify an opaque identifier for any request using the `X-Opaque-Id` header. This identifier is used to track tasks and deduplicate deprecation warnings in server-side logs. This identifier is used to differentiate between callers sending requests to your OpenSearch cluster. Do not specify a unique value per request.
+
+#### Example request
+
+The following request adds an opaque ID to the request:
+
+```json
+curl -H "X-Opaque-Id: my-curl-client-1" -XGET localhost:9200/_tasks
+```
+{% include copy.html %}