docs: Update info about metrics for 2.6

Also add events.handlers namespace and fix dhall link.

src/docs/markdown/caddyfile/options.md

@@ -105,6 +105,7 @@ Possible options are:
 			write       <duration>
 			idle        <duration>
 		}
+		metrics
 		max_header_size <size>
 		log_credentials
 		protocols [h1|h2|h2c|h3]
@@ -381,6 +382,11 @@ listener_wrappers {
 - **idle** is a [duration value](/docs/conventions#durations) that sets the maximum time to wait for the next request when keep-alives are enabled. Defaults to 5 minutes to help avoid resource exhaustion.
 
 
+##### `metrics`
+
+Enables Prometheus metrics collection; necessary before scraping metrics. Note that metrics reduce performance on really busy servers. (Our community is working on improving this. Please get involved!)
+
+
 ##### `max_header_size`
 
 The maximum size to parse from a client's HTTP request headers. It accepts all formats supported by [go-humanize](https://github.com/dustin/go-humanize/blob/master/bytes.go).