Added ability to instrument when exceptions happen

Occasionally, the instrumented code will raise an exception, and we
would still want the subscribers to be notified. For example, if an HTTP
client fails by raising an exception, a subscriber doing logging should
still be able to log the failed request.

```
instrument("example.request") do
  client.post("http://url.that.fails.example/")
end
```

This will call the subscriber with an event that contains the exception
within the payload under a key `:exception`. This implementation also
re-raises the original error, so the callstack is unchanged.

lib/dry/monitor/notifications.rb

@@ -41,7 +41,12 @@ def stop(event_id, payload)
 
       def instrument(event_id, payload = EMPTY_HASH)
         result, time = @clock.measure { yield payload } if block_given?
-
+      # We should always try to instrument, even the system-level exceptions
+      rescue Exception => e # rubocop:disable Lint/RescueException
+        payload = {} if payload.equal?(EMPTY_HASH)
+        payload[:exception] = e
+        raise
+      ensure
         process(event_id, payload) do |event, listener|
           if time
             listener.(event.payload(payload.merge(time: time)))

spec/integration/instrumentation_spec.rb

@@ -63,5 +63,28 @@
         outside_block: true, inside_block: true
       )
     end
+
+    MyError = Class.new(StandardError)
+    it 'still notifies when the instrumented block raises an exception' do
+      captured = []
+
+      notifications.subscribe(:sql) do |event|
+        captured << event
+      end
+
+      expect {
+        notifications.instrument(:sql) do
+          @line = __LINE__ + 1
+          raise MyError
+        end
+      }.to raise_error(MyError)
+
+      expect(captured).to_not be_empty
+      exception = captured.first.payload[:exception]
+      expect(exception).to match kind_of(MyError)
+      # verify the exception backtrace comes from the instrumented code, not
+      # the `#instrument` method
+      expect(exception.backtrace.first).to start_with("#{__FILE__}:#{@line}")
+    end
   end
 end