Add metrics to rate limiter

lib/music_library_web/telemetry.ex

@@ -69,6 +69,16 @@ defmodule MusicLibraryWeb.Telemetry do
         ]
       ),
 
+      # Rate Limiter
+      summary("req.rate_limiter.throttle.sleep_ms",
+        unit: :millisecond,
+        description: "Time spent waiting for rate limit cooldown",
+        tags: [:name],
+        reporter_options: [
+          nav: "External APIs"
+        ]
+      ),
+
       # Assets
       summary("music_library.assets.cache_size",
         unit: {:byte, :kilobyte},

lib/req/rate_limiter.ex

@@ -56,6 +56,12 @@ defmodule Req.RateLimiter do
           remaining = cooldown - elapsed
 
           if remaining > 0 do
+            :telemetry.execute(
+              [:req, :rate_limiter, :throttle],
+              %{sleep_ms: remaining},
+              %{name: name}
+            )
+
             Process.sleep(remaining)
           end