MINOR: sample: Add UUID-fetch

Adds the fetch uuid(int). It returns a UUID following the format of
version 4 in the RFC4122 standard.

New feature, but could be backported.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index ba5a8b3..ee9712f 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -14316,6 +14316,11 @@
   needed to take some routing decisions for example, or just for debugging
   purposes. This random must not be used for security purposes.
 
+uuid([<version>]) : string
+  Returns a UUID following the RFC4122 standard. If the version is not
+  specified, a UUID version 4 (fully random) is returned.
+  Currently, only version 4 is supported.
+
 srv_conn([<backend>/]<server>) : integer
   Returns an integer value corresponding to the number of currently established
   connections on the designated server, possibly including the connection being