Pairing: Add missing telemetry metrics

Add metrics that were not imported when merging Pairing and Pairing API.

Signed-off-by: Davide Briani <davide.briani@secomind.com>

Author: davidebriani

apps/astarte_pairing/lib/astarte_pairing_web/telemetry.ex

@@ -111,6 +111,17 @@ defmodule Astarte.PairingWeb.Telemetry do
       ),
       sum("astarte.pairing.request.response_body_bytes",
         tags: [:realm]
+      ),
+      counter("astarte.pairing.register_new_device.count",
+        tags: [:realm],
+        description: "Pairing requests total count"
+      ),
+      counter("astarte.pairing.get_credentials.count",
+        tags: [:realm],
+        description: "Get credentials requests total count"
+      ),
+      last_value("astarte.pairing.service.health",
+        description: "Service state: 1 if good, 0 if not."
       )
     ]
   end