Link to the data model from naming.

Make it easier to find the list of valid characters in metric names by
linking the data model page from the naming best practices page.
Signed-off-by: Ben Kochie <superq@gmail.com>

content/docs/practices/naming.md

@@ -14,6 +14,7 @@ practices, e.g. naming conventions, differently.
 
 A metric name...
 
+* ...must comply with the [data model](/docs/concepts/data_model/#metric-names-and-labels) for valid characters.
 * ...should have a (single-word) application prefix relevant to the domain the
   metric belongs to. The prefix is sometimes referred to as `namespace` by
   client libraries. For metrics specific to an application, the prefix is