Fix counters not decrementing when the process is exited

This fixes the issue by trapping exits for the duration of the fun,
then turning exit trapping off and killing the process if there is an
exit message in the mailbox.

The real-world case where this fixes things is Pleroma MediaProxy.
Without exit trapping, the process would get killed if the connection
was closed client-side and a counter would be left incremented.

I am not sure if trapping exits is the optimal solution, but I don't
see any other option.

lib/concurrent_limiter.ex

@@ -101,25 +101,50 @@ defmodule ConcurrentLimiter do
 
     cond do
       counter <= max_running ->
-        :telemetry.execute([:concurrent_limiter, :execution], %{counter: counter}, %{limiter: name})
+        :telemetry.execute([:concurrent_limiter, :execution], %{counter: counter}, %{
+          limiter: name
+        })
+
+        Process.flag(:trap_exit, true)
+
         try do
           fun.()
         after
           dec(ref, name)
+          Process.flag(:trap_exit, false)
+
+          receive do
+            {:EXIT, _, reason} ->
+              Process.exit(self(), reason)
+          after
+            0 -> :noop
+          end
         end
 
       counter > max ->
-        :telemetry.execute([:concurrent_limiter, :overload], %{counter: counter}, %{limiter: name, scope: "max"})
+        :telemetry.execute([:concurrent_limiter, :overload], %{counter: counter}, %{
+          limiter: name,
+          scope: "max"
+        })
+
         dec(ref, name)
         {:error, :overload}
 
       retries + 1 > max_retries ->
-        :telemetry.execute([:concurrent_limiter, :max_retries], %{counter: counter}, %{limiter: name, retries: retries + 1})
+        :telemetry.execute([:concurrent_limiter, :max_retries], %{counter: counter}, %{
+          limiter: name,
+          retries: retries + 1
+        })
+
         dec(ref, name)
         {:error, :overload}
 
       counter > max_running ->
-        :telemetry.execute([:concurrent_limiter, :wait], %{counter: counter}, %{limiter: name, retries: retries + 1})
+        :telemetry.execute([:concurrent_limiter, :wait], %{counter: counter}, %{
+          limiter: name,
+          retries: retries + 1
+        })
+
         wait(ref, name, fun, wait, opts, retries + 1)
     end
   end