:circuit_breaker plugin: added support for .on_circuit_open callback

called when a circuit is open.

```ruby
HTTPX.plugin(:circuit_breaker).on_circuit_open do |req|
  # ... do smth
end

lib/httpx/plugins/circuit_breaker.rb

@@ -31,6 +31,16 @@ module HTTPX
           @circuit_store = orig.instance_variable_get(:@circuit_store).dup
         end
 
+        %i[circuit_open].each do |meth|
+          class_eval(<<-MOD, __FILE__, __LINE__ + 1)
+        def on_#{meth}(&blk)   # def on_circuit_open(&blk)
+          on(:#{meth}, &blk)   #   on(:circuit_open, &blk)
+        end                    # end
+          MOD
+        end
+
+        private
+
         def send_requests(*requests)
           # @type var short_circuit_responses: Array[response]
           short_circuit_responses = []
@@ -59,6 +69,12 @@ module HTTPX
         end
 
         def on_response(request, response)
+          emit(:circuit_open, request) if try_circuit_open(request, response)
+
+          super
+        end
+
+        def try_circuit_open(request, response)
           if response.is_a?(ErrorResponse)
             case response.error
             when RequestTimeoutError
@@ -69,8 +85,6 @@ module HTTPX
           elsif (break_on = request.options.circuit_breaker_break_on) && break_on.call(response)
             @circuit_store.try_open(request.uri, response)
           end
-
-          super
         end
       end
 

sig/plugins/circuit_breaker.rbs

@@ -5,7 +5,7 @@ module HTTPX
       class CircuitStore
         @circuits: Hash[String, Circuit]
 
-        def try_open: (generic_uri uri, response response) -> void
+        def try_open: (generic_uri uri, response response) -> response?
 
         def try_respond: (Request request) -> response?
 
@@ -30,7 +30,7 @@ module HTTPX
 
         def respond: () -> response?
 
-        def try_open: (response) -> void
+        def try_open: (response) -> response?
 
         def try_close: () -> void
 
@@ -52,6 +52,10 @@ module HTTPX
 
       module InstanceMethods
         @circuit_store: CircuitStore
+
+        private
+
+        def try_circuit_open: (Request request, response response) -> response?
       end
 
     end

test/support/requests/plugins/circuit_breaker.rb

@@ -70,6 +70,30 @@ module Requests
         assert response1 == response2
       end
 
+      def test_plugin_circuit_breaker_on_circuit_open
+        return unless origin.start_with?("http://")
+
+        unknown_uri = "http://www.qwwqjqwdjqiwdj.com"
+
+        circuit_opened = false
+        session = HTTPX.plugin(:circuit_breaker,
+                               circuit_breaker_max_attempts: 1,
+                               circuit_breaker_break_in: 2,
+                               circuit_breaker_half_open_drip_rate: 1.0)
+                       .on_circuit_open { circuit_opened = true }
+
+        # circuit closed
+        response1 = session.get(unknown_uri)
+        verify_error_response(response1)
+
+        # circuit open
+        response2 = session.get(unknown_uri)
+        verify_error_response(response2)
+        assert response2 == response1
+
+        assert circuit_opened
+      end
+
       # def test_plugin_circuit_breaker_half_open_drip_rate
       #   unknown_uri = "http://www.qwwqjqwdjqiwdj.com"