Added ability to modify payload within block

When instrumenting a block of code, it is sometimes useful for that
block to add more things to the payload. A common example is when
performing an http request, it is useful to include the response status
code in the instrumentation.

```
instrument("example.request", url: url, body: body) do |payload|
  response = http.post(url, params: body)
  payload[:response] = response
end
```

This change simply yields the payload to the block to enable this.

NOTE: If the instrument method is called with no overriding payload, the
payload yielded to the block is the default EMPTY_HASH that is frozen,
which results in a FrozenError if the block attempts to add anything to
the hash.

lib/dry/monitor/notifications.rb

@@ -40,7 +40,7 @@ def stop(event_id, payload)
       end
 
       def instrument(event_id, payload = EMPTY_HASH)
-        result, time = @clock.measure { yield } if block_given?
+        result, time = @clock.measure { yield payload } if block_given?
 
         process(event_id, payload) do |event, listener|
           if time

spec/integration/instrumentation_spec.rb

@@ -47,5 +47,21 @@
 
       expect(captured).to eql([[:sql]])
     end
+
+    it 'yields the payload to the instrumented block' do
+      captured = []
+
+      notifications.subscribe(:sql) do |event|
+        captured << event
+      end
+
+      notifications.instrument(:sql, outside_block: true) do |payload|
+        payload[:inside_block] = true
+      end
+
+      expect(captured[0].payload).to match hash_including(
+        outside_block: true, inside_block: true
+      )
+    end
   end
 end