Add a note about when metrics was added

src/docs/markdown/metrics.md

@@ -9,6 +9,9 @@ Caddy server on an embedded device, it's likely that at some point you'll want
 to have a high-level overview of what Caddy is doing, and how long it's taking.
 In other words, you're going to want to be able to _monitor_ Caddy.
 
+> **Note**
+> This was added in Caddy 2.6
+
 ## Enabling metrics
 
 You'll need to turn metrics on. If using a Caddyfile, enable metrics [in global options](/docs/caddyfile/options#metrics):