chore: Document approx_topk keyword. (backport release-3.3.x) (#15347)

Co-authored-by: Karsten Jeschkies <[email protected]>

docs/sources/query/metric_queries.md

@@ -153,3 +153,14 @@ Examples:
       or
     vector(0) # will return 0
     ```
+
+## Probabilistic aggregation
+
+The `topk` keyword lets you find the largest 1,000 elements in a data stream by sample size. When  `topk` hits the maximum series limit, LogQL also supports using a probable approximation; `approx_topk`  is a drop-in replacement when `topk` hits the maximum series limit.
+
+```logql
+approx_topk(k, <vector expression>)
+```
+
+It is only supported for instant queries and does not support grouping. It is useful when the cardinality of the inner
+vector is too high, for example, when it uses an aggregation by a structured metadata label.